"In my stars I am above thee; but be not afraid of greatness: some are born great, some achieve greatness, and some have greatness thrust upon 'em."
Part of the play's plot is to take the arrogant Malvolio down a peg or two. So a phony love letter (purportedly written by Olivia to Malvolio) is written by some mischievious characters and left for Malvolio to find, which he does and reads it outloud. The letter relates how much Olivia loves Malvolio and what he can do to show her that he loves her, as well. This particular line is is telling Malvolio that he can become "great" by doing certain things, each of which is more idiotic than the last. While Olivia isn't in love with Malvolio, he goes on to behave like a complete fool and Olivia eventually locks him up, believing he has gone insane.