Can anyone explain reactions?

[Mollerz]

I am new to DnD and don;t get it.

[LinkToReality.ttv]

If you mean the action type called Reactions it's a type of ability you can use one per round, but you can use it on any character's turn that round.
It can be things like Attack of Opportunity, or Hellish Rebuke, or Deflect Missiles. Basically things that trigger when a certain criteria is fulfilled.

Edit: I think that by default the first possible reaction you can make will happen automatically, but there's a setting in the options to make it pop up a prompt asking you if and what reaction you wanna use.

[Yojo0o]

Any creature in DnD has one "Reaction" per round of combat. It's a combat resource, like your action and bonus action, but with some important differences: You need a specific trigger to allow you to use a reaction (such as an enemy leaving your melee range triggering Attack of Opportunity), and it doesn't need to be your turn to use your reaction.

[Mollerz]

Originally posted by Yojo0o:

Any creature in DnD has one "Reaction" per round of combat. It's a combat resource, like your action and bonus action, but with some important differences: You need a specific trigger to allow you to use a reaction (such as an enemy leaving your melee range triggering Attack of Opportunity), and it doesn't need to be your turn to use your reaction.

Oh got ya. That is actually a pretty cool feature. I haven't played DnD since 2E when I was a kid so wasn't aware of it.