BOOKBABY - A “beat” is a description of the physical action a character makes while speaking, and good beats can bring your characters to life and make your dialogue pop right off the page.
Beats can also help you show your readers instead of telling them. (Misuse of show, not tell is a common mistake many first-time authors make. Remember that readers don’t like to be told what to think!)
Read more at Using Beats to Bring Your Dialogue to Life: Turn the Beat Around.