First, it must be noted that Speed is a servant, who is addressing his master, Valentine. Traditionally, servants are to do as they are told and are often not as intelligent or witty as their masters. Not in this case.
Valentine is quite a handsome fellow in this play, but not the brightest bulb on the block. And this is exactly why Speed is carrying on in this speech. He can't believe that his master doesn't get the "jest" played on him by Silvia.
Speed has just witnessed the woman that this master loves, Silvia, make her "moves" on Valentine. She has had him write letters in her name to a "secret nameless friend." These are, in fact, love letters meant for Valentine from Silvia. She tries to return them to Valentine so that he will understand that she has "written" of her love TO HIM. If this seems confusing, then you will have some sympathy for why Valentine doesn't get it.
But Speed does. After Silvia leaves, he uses the above lines to comment on the fact that Silvia, in her boldness, is playing the man's part in this exchange (the suitor) and that Valentine has become the one pursued (or "sue"-ed). Traditional roles are turned on their ear in this speech: The suitor becomes the "sued," and the pupil, the teacher.
For Speed, a real whiz kid of a servant, all this is as plain as the "nose" on your "face" (or "a weathercock on a steeple"). His ability to perceive this jest, incidentally, provides another turn about in which the servant outsmarts the master. This sort of wise-cracking servant is actually a very common occurrence, and used frequently for comic effect, in Shakespeare's comedies.