Mr Amulet,
There are two types of trading possibilities; going long or going short. Going long means you think the value of whatever you are interested to trade will increase in the future, so you buy them to sell later, simple. When you go short you think the value of whatever you are interested in will decrease, so you use your margin account in order to sell what you don't own yet, then no matter what happens you must cover your short position, that is to buy back the amount you sold.
Cheers!