Cover

Walls, trees, creature, and other obstacles can provide cover, making a target more difficult to harm. As detailed in the Cover table, there are three degrees of cover, each of which gives a different benefit to a target.

A target can benefit from cover only when an attack or other effect originates on the opposite side of the cover. If a target is behind multiple sources of cover, only the most protective degree of cover applies; the degrees aren't added together. For example, if a target is behind a creature that gives Half Cover and a tree trunk that gives Three-Quarters Cover, the target has Three-Quarters Cover.

Cover

Degree	Benefit to Target	Offered By...
Half	+2 Bonus to AC and Dexterity saving throws	Another creature or an object that covers at least half of the target
Three-Quarters	+5 bonus to AC and Dexterity saving throws	An object that covers at least three-quarters of the target
Total	Can't be targeted directly	An object that covers the whole target

Unseen Attackers and Targets

When you make an attack roll against a target you can't see, you have Disadvantage on the roll. This is true whether you're guessing the target's location or targeting a creature you can hear but not see. If the target isn't in the location you targeted, you miss.

When a creature cant' see you, you have Advantage on attack rolls against it.

If you are hidden when you make an attack roll, you give away your location when the attack hits or misses.