A beat is acting jargon for a natural division within a larger structure such as a scene. For example, a scene where two people meet at a restaurant and then break up might have three beats: the part where they arrive at the restaurant and order their food, the part where they start airing their grievances, and the part where they break up and threaten to tell each other's secrets and then decide to call a truce. Often when you are memorizing or rehearsing a scene, you do just one beat at a time.
A Harold has three large beats, each about 8-10 minutes long, each of which contains many short scenes.
A beat is also a half second or so--usually a moment when nobody speaks. For example, "I'm going to have a baby." Beat. "Is it mine?"