I'm sure the lawyer-types floating about could explain it better than I... But freedom of speech in the US basically means you're free to express yourself without government censorship/involvement. Where in some cultures there have been times (and still are in places) where, say, speaking out against your country's leader can get you imprisoned (or worse). Freedom of speech says 'You have a right to your own thoughts, opinions, and counsel, without the government's influence.'
A lot of people -- especially kids whose schools haven't taught it properly -- think freedom of speech means 'I can say whatever I want whenever I want and no one can stop me.'
For the kids, on the forums I worked on, I'd often explain it as:
'If someone came into your home and began insulting you, you'd ask them to stop or leave. You wouldn't just stand there and allow it to happen. This site is privately owned and run. We want it to be a safe environment for everyone, so sometimes we have to ask you to keep those thoughts to yourself... or to leave.'
It dumbs the concept down a lot, yes, but... they were kids.