The prologue serves as an introduction to events which are about to unfold on stage. It is similar to a thesis statement in an essay and is much like a preview to a movie to pique an audience's interest--Shakespearian style. The prologue serves to whet the audience's appetite.
Benvolio makes the following statement:
"The date is out of such prolixity:
We'll have no Cupid hoodwink'd with a scarf,
Bearing a Tartar's painted bow of lath,
Scaring the ladies like a crow-keeper;
Nor no without-book prologue, faintly spoke
After the prompter, for our entrance:"
What he is saying is that their arrival at the Capulet's masked ball will be unannounced, unlike when Cupid, bearing a toy-bow made a speech to introduce a group of ladies to an audience. Their arrival will be intrusive and since they are enemies of the Capulets, they would not want their arrival to be made public. Also, because they will be in disguise, no one will recognise them and their arrival would not, therefore, be announced anyhow.
Benvolio is referring to the general nature of plays where an actor appears on stage assisted by a prompter who reads from a book (obviously the text of a play) to help the actor should he/she forget his/her lines. In this instance, however, there would be no such prompter or book.
Furthermore, he is also making an indirect reference to the prologue of this play, where the audience is informed. Their audience, however, will be unprepared and would therefore not know what to expect.