Top of Show refers to a set amount of money that is budgeted in a show for its guest star roles. For example, a role might say that the show “does not break top of show,” which means that no matter who the actor is, the show will not pay that person more than what their top of show rate is.
Top of show rates vary on the type of show (1 hour, ½ hour), and the type of platform. More well-known actors are typically offered more than top of show (assuming the show breaks top), and may be offered rates such as “double top,” i.e. twice the show’s top of show rate.