Laertes, a young lord about to depart for Paris, has some dear parting words with his sister, Ophelia.
He asks her about her relationship with Prince Hamlet, and says that because Hamlet is way above her (in the social status way), he might have to marry someone else for the sake of the state.
So, he's worried that Hamlet might take advantage of her. He warns her that if she has sex with Hamlet, she'll be damaged goods.
Ophelia is no wimp, so she tells Laertes not to be a hypocrite. In other words, he's probably been sleeping around pre-maritally himself.
Their dad enters and gives his son a lot of long-winded advice: listen more than he talks; to not borrow or lend money; not to bling himself out (he actually says not to be gaudy); and, famously, "This above all, to thine own self be true."
When Laertes finally leaves, Polonius tells his daughter that Hamlet only wants to sleep with her. He also tells Ophelia she's an idiot if she actually believes Hamlet loves her. He orders her to stop seeing him and uses the word "pooh" in the process. She agrees.
Be sure to check out this scene, staged by the awesome actors at This is Hamlet.