Certain situations allow you to take a special action called a reaction. A reaction is an instant response to a trigger of some kind, which can occur on your turn or on someone else's turn. The most common type of reaction is an opportunity attack (see Opportunity Attacks).

Once you have taken a reaction, you can't take another one until the start of your next turn. If the reaction interrupts another creature's turn, that creature can continue its turn right after the reaction.

Spending a spirit point doesn't require any sort of action or reaction: fate can smile upon you at any time, even when it isn't your turn!