Reaction

Reactions are quick, instinctual responses triggered by events in the game world.
A reaction is an instant response to a trigger of some kind, which can occur on your turn or on someone else's, both in and out of combat. Regardless of the game mode, you can only react to one event every six seconds. You must be able to sense a trigger in order to react to it, either with sight, hearing, smell, or touch.
Whenever you have only a brief moment to respond to something unexpected, if you have not already reacted to something in that six-second round, you can "use" your reaction to formulate a split-second response.
The GM describes what you are aware of and asks if you would like to use a reaction. If so, the player first declares, and then describes their reaction. The GM determines if the reaction could avoid or mitigate the unexpected event, whether it can succeed, and whether it can fail. The GM then asks for a saving throw, ability check, or some other action based on the circumstance. Any spell with a casting time of one Reaction can be used in this instance.

Example: Carol is trying to sneak past a patrolling Goblin to steal a work of art. The goblin is walking ahead of her and she’s creeping silently behind, several feet back. Unbeknownst to Carol, she has failed her Dexterity (Stealth) check and the goblin is about to turn around because he thought he heard her breathing. “Suddenly, the goblin freezes. He’s about to turn, what do you do?” Carol thinks for a minute. “Is the cave wall irregular enough? Can I hide in the folds and crevices?” The GM says, “You could, but you’ll have Disadvantage since you have to move fast.” Carol, who has mastered the Quick Draw Maneuver thinks and says, “I’ll whip my Dagger and try to take out the goblin before he turns.” The GM says “okay, give me a Ranged Attack roll with Advantage since the goblin is currently unaware of you."

When you are granted a saving throw based on an unexpected occurrence, the quality of your reaction can grant Advantage on the saving throw. You do not always know what reaction would grant you an advantage on your saving throw, and in those cases you must guess what reaction to take. After reactions are declared, the GM decides whether the reactions taken help you on your saving throws. If the reaction taken does help you, then you receive advantage on your saving throw.
In situations where a character would not normally receive a saving throw, a helpful reaction can grant a special saving throw.

Example: A player has stepped on a pressure plate that will trigger a 'no saving throw' trap-door opening underneath the forward rank members of the party, Alice and Bob. The GM tells the party “Alice, as your foot hits the ground, you feel a tile give way and hear an audible click. You’ve triggered a pressure plate. Do you want to use your reaction? What do you do?” Alice, not knowing what is coming, says “Yes. I tumble forwards, trying to roll out of the way.” Bob, also unaware of what is coming, says “I raise my shield and stand my ground, gritting my teeth and trying to absorb whatever is about to hit me.” The GM determines that Alice’s action could conceivably carry her forward away from the Pit Trap. He asks her to make a Dexterity Saving Throw. Bob’s action, however, won’t do him any good. He plunges into the pit.

When you declare a reaction that involves an instantaneous movement, such as diving out of the way of a Fireball, move your token to the appropriate space. For example, if you want to dive to the ground, you drop Prone. If you leap to the left or right, move your character to the nearest appropriate space. If there are multiple options, you choose which space.

Reactions in Combat

Combat is fast-paced and hectic, and unexpected triggers are a common occurrence.
Therefore, many special abilities, spells, and situations in Combat allow you to take reactions with their own definitions that differ from the basic rules above. The Opportunity Attack is one common type of combat reaction.
When you take a reaction in combat, you can't take another one until the start of your next turn -- including the non-combat reactions described above. If your reaction interrupts another creature's turn, that creature can continue its turn immediately after you take your reaction.