This is one of those holdovers from old Hollywood when writers would use the word “beat” (usually in parentheses) to indicate a pause in the action, such as when a character doesn’t reply to a question in dialogue. Now it’s just another way for inexperienced screenwriters to micromanage the flow of their scripts. Sometimes people write “(beat)” on its own line to indicate a pause, and other times people write something like, “Jane stares at William for a beat and then walks away.” There’s never a good reason to do either of these things.
If you want to pause the action, well, you shouldn’t. Just write out the salient descriptions and dialogue and then trust your outstanding word choices to let the reader know the pacing. Any way that you write it, using a “beat” to pause your script will only hurt it. Just stick with telling the story in plain, everyday English.