Silvius is a young shepherd who is madly love with Phoebe, a snobby shepherdess who thinks she's way too good for Silvius. Like Romeo at the beginning of Romeo and Juliet, Silvius is a cliché and also a parody of the classic "Petrarchan lover," which is basically a guy who falls in love with a woman he has no chance of hooking up with. (Petrarch was a 14th-century Italian poet – his sonnets were super-popular in Shakespeare's day.) Typically, the "Petrarchan lover" spends most of his time complaining about the cruelty of a disdainful mistress who can kill a man with a dirty look but who is also very beautiful. (Most of the time, the mistress has eyes like stars, lips like cherries, hair like silk, and breasts like melons – no kidding.)
Even though love-struck Silvius thinks Phoebe's the most beautiful woman on the planet, Rosalind informs her that she isn't all that: "You have no beauty," says Rosalind. "Tis not your inky brows, your black silk hair,/ Your bugle eyeballs, nor your cheek of cream,/ That can entame my spirits to your worship" (3.5.1). Oh, snap! Basically, Rosalind tells Phoebe that she should stop her wannabe "Petrarchan mistress" routine because nobody's going to be writing poems about her beauty any time soon. In fact, Rosalind also says that Phoebe should grab Silvius while she can because nobody else will want to marry her: "For I must tell you friendly in your ear,/ Sell when you can: you are not for all markets" (3.5.1).
If all of this seems rather mean-spirited and cruel, we should remember a couple of things. 1) Phoebe treats Silvius like garbage, so she sort of has it coming. 2) Shakespeare is making fun of all poser couples who try to act like Petrarch's so-called "ideal" lovers. So, when Rosalind bags on Silvius and Phoebe, she's rejecting their silly "love" games and is arguing for something more genuine.