It's not a button, it's a toggle.
In 5th edition each players receives a reaction per round they can use for certain events - each triggered by specific circumstances. Larian have currently implemented a system where it will display all of the possible reactions for that character, ranging from things like absorb elements (when added) to hellish rebuke. You can choose whether or not you toggle them on so if they trigger, they just trigger (unlike in 5th edition when you could choose).
It simplifies reactions in an effort to speed things up, or so Larian tell us, but unfortunately means if you are hit, you don't get to choose whether to shield it, and potentially stop it, or hellish rebuke once you take the damage. (I know shield isn't in the game... yet).