Others will likely see it other or a number of ways. (Which is cool too.)
A beat is a pause.
A BEAT is like a unit of action/reaction story dynamic in a scene.
(It's likely explained in various ways in a number of approaches to writing,
but some will say a BEAT leads up to or precipatates the turning point in a scene.)
Not the best description (Sorry), but somone here will put it into better words for you.
-Thanks-