Noob futures question

I keep reading that futures are settled every day, i.e. if your position goes up, that money is put into your account at the end of the day, and vice versa if you lose money. How does this work? Where does the money come from is you haven't yet sold your contracts?

 

http://en.wikipedia.org/wiki/Mark_to_market

 

Mark to market...i couldnt think of that phrase in my googling. Thank you.