Stealth

The rulebooks never give a precise definition of hiding. There is no “hidden condition” - the best definition of “being hidden” is the following;

“Your opponent either doesn’t know that you are there, doesn’t pay any attention to you, or doesn’t know where exactly you are located”.

Using this broad definition works well with all of the rules as presented. It also means that a PC could be hidden even if all your foe had to do is to look in your direction to see you. When they did, the PC would no longer be hidden. It also means that when you are successful at being stealthy, it has the same effect as being hidden.
The main rule in the Player’s Handbook for hiding is:

“You can’t hide from a creature that can see you.”

This sounds like it is saying that you must either be in a heavily obscured area or have total cover to even attempt to hide. I contend that this is not correct. It only means that whoever you are attempting to hide from is not looking in your direction (the DM has the final say on this).
“Being stealthy” is trying to remain undetected which is the same as trying to hide. Examples abound in the Player’s Handbook to support this idea. In the section on surprise, the terms “be stealthy” and “hiding” are used to mean the same thing. In the section on noticing threats “hidden threats” obviously includes “a stealthy creature following the group”. On the section on stealth – traveling at a slow pace, it says to refer to the rules on hiding when trying to “surprise or sneak by other creatures.” In the section on perception “hear monsters moving stealthily in the forest,” “orcs lying in ambush on a road,” and “thugs hiding in the shadows of an alley” are all examples of creatures that your Wisdom (Perception) check lets you detect. And in the section on stealth “Make a Dexterity (Stealth) check when you attempt to conceal yourself from enemies, slink past guards, slip away without being noticed, or sneak up on someone without being seen or heard.” Which are all examples of being hidden.

So when can I attempt to hide?

You can attempt to hide whenever the creature or creatures you are attempting to hide from can’t see you. You could be invisible. (Being hidden is different from the “Invisible” condition in that you can be invisible and still not be hidden if your opponent can tell where you are by hearing you or by some other means.) Or you could be on the opposite side of anything that provides total cover, or in a heavily obscured area (such as darkness if your foe doesn’t have darkvision), or your foe could be distracted (if the DM agrees). You can also attempt to hide if you are in a lightly obscured area if you have the Skulker feat.
With the wood elf’s “Mask of the Wild” ability you can attempt to hide even when you are only lightly obscured by foliage, heavy rain, falling snow, mist, and other natural phenomena. From the wording, I take it to mean that you can’t use this ability to attempt to hide in dim lighting (although your DM might allow it), but you can in the area of effect of an insect plague.
With the lightfoot halfling’s “Naturally Stealthy” ability you can attempt to hide even when you are obscured only by a creature that is at least one size larger than you. You would have to first move to a position that placed that creature between you and the creature you are hiding from.
The Rogue’s “Cunning Action” that allows him to take a hide action as a bonus action each round, does not release him from the need to meet at least one of the above requirements before attempting to hide.

How do I hide?

As a hide action in combat, or any time you attempt to hide, you make a Dexterity (Stealth) check and write down that number. As long as you remain in hiding, if any creature has a chance to detect your presence, their Passive Wisdom (Perception) score must beat your stealth check. I would rule that if you are hiding and cannot be seen and are silent the creatures would normally have no chance to detect you. If a creature is actively trying to locate you, compare your check to a Wisdom (Perception) check that the creature makes at that time. If you cannot be seen, or if you are in an area that is lightly obscured, they have disadvantage on the check.

What benefits do I receive from being hidden?

If you are hidden before the first round of combat you can surprise your opponents and get a free round to attack them before they can react. (You are no longer hidden after you attack.)
On all attacks against you, the attacker must first identify where he thinks you are located. The attack will automatically miss if you are not in that 5 foot area. If you are in that area, the attack is made with disadvantage on the attack roll. The DM should require a roll with disadvantage, even if you are not in the targeted area and simply tell the attacker that his attack missed.
If you are hidden you make attacks with advantage. However, you will no longer be hidden if the attack hits or misses.

When am I no longer hidden?

You can come out of hiding at any time of your choosing. You are no longer hidden if you attack someone even if the attack misses (exception: if you have the Skulker feat, attacking with a ranged weapon and missing doesn’t reveal your position).
If you move to a location where your opponent can see you, or if your opponent moves into a position where he can see you, or if the object or creature that was providing your total cover moves or is no longer providing cover for some reason, if you make a noise, or do anything that could give away your position, the creature you are hiding from can make another Wisdom (Perception) check to attempt to detect you.
If you move from a heavily obscured area to a lightly obscured area you can try to continue to hide but the creatures you are hiding from get a Wisdom (Perception) check to detect you.
Once you are no longer hidden your opponents will know where you are so they no longer have to guess where to attack. But if you can still not be seen (if you are invisible, for example), attack rolls against you have disadvantage, and your attack rolls still have advantage.

If I am hiding behind a tree, can I stand out and attack with my ranged weapon with advantage and then return to hiding on my round of combat?

It depends. If you are doing this during a fight, it is assumed that all the creatures in the fight are alert and aware their surroundings, so they get a Wisdom (Perception) check to spot you when you move out from behind total cover. If they succeed you are no longer hiding so you don’t get advantage to the attack. However, if the fight hasn’t started yet, you have a chance to surprise them as long as they aren’t looking in your direction. In that case you an attack with advantage, but you will no longer be hidden as soon as you attack. If you are a 2nd level or higher rogue you can use a bonus action to attempt to hide again. But remember, if they see you duck behind a tree, they have a good guess at where you are hiding. In that case, when you stuck your head out I would give them advantage on their perception check – or give them an automatic success, depending on the circumstances.
If it makes logical sense, go for it just remember,  “hide” is not a magical condition.