What is a beat sheet?
A beat sheet is a document used to prepare the essential dramatic beats (i.e the dramatic structure) of a story: what happens, when and why. Individual scenes can also be broken down into singular beats of action, while the actual content within the scenes (the dramatised action) would make up a 'scene by scene breakdown' before going to script.
More on beat sheets:
Beat Sheets, explained
Beat Sheets, revisited (with analysis scene from The Graduate)