Just before departing for his studies in Paris, Laertes gives some unsolicited advice to his sister, Ophelia. He tells her that Hamlet's attentions are not to be taken seriously; he's just going through a typical phase of hot-blooded youth. And besides, even if Hamlet really is in love with Ophelia, as a member of the royal family, he's not in a position to make his own decisions concerning matters of the heart; he has family responsibilities to consider.
Taking everything into consideration, Ophelia should be on her guard against Hamlet, Laertes advises. Even if he professes his undying love for her, she should remain cautious. Furthermore, it would be shameful for Ophelia to allow Hamlet to open her "chaste treasure" (i.e., take her virginity). Even good girls can get a bad reputation, and that's the last thing Laertes wants for his little sister.
As for Polonius, he tells Ophelia that Hamlet's professions of love mean absolutely nothing. His affections are not real, and if Ophelia believes otherwise, then she's nothing more than a "baby." Even if Hamlet has made vows to Ophelia, they are but "springes to catch woodcocks" (traps to catch birds). When men's hearts are on fire, they're apt to say all kinds of things they don't really mean. Polonius warns Ophelia not to take such expressions at face value or regard them as evidence of true love.
As always, Polonius is thinking of himself and his own interests rather than those of his daughter. If Ophelia believes Hamlet's expressions of love, then she runs the serious risk of turning her father into a laughingstock. Polonius's position at court is everything to him, and the last thing he wants is to have his exalted status undermined by his daughter's foolish actions.