This witty exchange occurs between Robin the clown and Wagner in Act I, scene four. To understand these lines, it helps to know what the clown and Wagner have been discussing. Basically, Wagner tries to convince the poor clown to be his servant for the next seven years.
Wagner gives the clown some money to secure his service, only the clown cannot tell what kind of currency it is when he says "Gridirons! What be they?"
Wagner attempts to explain to the clown that the coins are "French crowns," a type of currency used in France. The clown sees absolutely no use in having French coins, however, only valuing "English counters" or English coins.
A comical scene ensues in which the clown tries to give the coins back to Wagner, only to have Wagner shove the coins back into his hands. The clown refuses to take the money until Wagner summons two devils to scare him. Then the clown concedes and allows himself to be hired. Wagner replies that he would like to be called "Master Wagner" from now on.