What do these lines from the letter Malvolio finds mean in Twelfth Night? "If this fall into thy hand, revolve. In my stars I am above thee, but be not afraid of greatness...."

The meaning of this short passage from Act II, Scene 5, of Twelfth Night is that of an encouragement to Malvolio that he strive to attain the love of Olivia.
In the previous scene which is part of the comedy and sub-plot, Maria hatches a scheme to wreak revenge upon Malvolio's Puritan personality by exploiting his desire for Olivia. As part of this scheme, she forges a letter that Malvolio believes is from Olivia; in it there is the suggestion that Olivia may be in love with him. This letter is dropped on the garden path so Malvolio will see it.
Sir Toby and Sir Andrew, who conspire with Maria, hide in the shrubbery so that they can observe and overhear Malvolio. As he approaches, Malvolio talks to himself, uttering the name of Olivia as though he is fantasizing about her. When he sees the letter lying on the garden path, Malvolio picks it up, believing it is from Olivia because Maria has sealed it with Olivia's sealing ring. To the delight of the hiding men, Malvolio reads the letter aloud. 
The passage that is cited above urges Malvolio to carefully ponder its contents if he happens to read it. The second sentence urges Malvolio to not be afraid of Olivia's "greatness," or social rank, which is above his. The suggestion here is that although Malvolio is beneath Olivia in social rank, he may still be able to be with her.
(Further in this scene Malvolio's opportunity to be with Olivia is encouraged more in the letter as "Olivia"--Maria pretending to be Olivia--urges Malvolio to rid himself of his lower-class trappings and to act more like a nobleman by following her suggestions in how to talk to others and what to wear. Of course, Maria causes Malvolio to be perceived by others not as one worthy of nobility, but instead, one to be ridiculed, and she gets her revenge.)