Triggers are game syncs that allow you to launch spontaneous contextual musical phrases called stingers. These phrases are superimposed and mixed over the music that is currently playing, as a reaction to what is happening in the game. The sync mechanism for triggers can use the tempo of the established music to make integration virtually seamless.
For example, the music currently playing could react to the game trigger "Main Player Hit Bullseye" by playing a drum fill over the current music while maintaining the current tempo. This drum fill is called a stinger.
Refer to the Wwise authoring application documentation for more information regarding triggers from the sound designer's point of view.
Refer to Integration Details - Triggers for an overview of how to integrate triggers in your game.