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 thanks her brother for the talk, but she also tells him he better not be a hypocrite, like so many people who are quick to give advice to others. In other words, if he's telling her not to sleep around outside of marriage, he better be holding himself to the same standard.
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 asks what they were talking about. When Ophelia said it was about Hamlet, Polonius doesn't beat around the bush.
He tells his daughter that Hamlet only wants to sleep with her. He also says she's an idiot if she actually believes any of Hamlet's lines. Polonius orders his daughter to stop seeing Hamelt, and Ophelia agrees to obey her father's wishes.
Be sure to check out this scene, staged by the awesome actors at This is Hamlet.