The storytelling concept of a "beat" was coined by Robert McKee in his book Story, and The Film Theorists do a great job of explaining what it is by using Return of the Jedi as an example. Check out the video below.

Now, Return of the Jedi isn't a film that most would think of when it comes to great screenwriting. However, the film does offer beginning screenwriters a good place to start in terms of educating themselves on basic story structure. And what better place to start than learning the building blocks of a narrative?

As it goes, beats make up scenes, scenes make up sequences, and sequences make up entire films. McKee describes a beat as the "smallest unit of change in a script," that means that a beat occurs when a character experiences a major emotional change within the story -- love to hate, happiness to sadness, etc.

This is really helpful to understand, because, if you haven't already noticed, writing a script can get overwhelming and confusing really quickly. But getting organized is definitely possible. Personally, I like to make a beat sheet, which is basically an outline of every beat in your screenplay. (Sometimes I make one after I make an overall outline for the story.) This helps me keep track of every reversal, including where they occur in the story, how they correspond with the story/character arcs, and whether or not they're pushing the story forward.

It has to be said -- Robert McKee isn't the only act in town. There are plenty of other screenwriting theories out there that don't incorporate, or even support, the idea of a beat. However, if McKee's approach works for you, reading up on what beats are is a great place to start learning about story structure.

Source: The Film Theorists