A beat is a pause in the storytelling.
It is also a term that can be used to describe the smallest significant unit of storytelling. Every line of dialogue, every choice, every establishing shot—they are all beats, in the sense of reviewing the story beat by beat… but when a Character “takes a beat” or the specific instruction (beat) is indicated, it is just an indicator that a moment of time passes.
The Game Engine will automatically keep track of the duration of beats that a player leaves between their actions, allowing for more nuanced replays.