In the vernacular of conjurers, magicians, and prestidigitators, patter is the constant talking of the magician while he/she performs a trick.
An example of typical patter:
"Ladies and gentlemen, if you will direct your attention to the deck of cards my lovely assistant is holding, you will see that they are by no means an ordinary deck of cards, being, as they are, made of cheese. You don't believe me? Allow me to demonstrate..."
and so on and so forth.
As well as keeping the audience entertained (patter usually being full of jokes), the purpose of patter is to distract the audience from whatever the magician is doing with his/her other hand.