In the forex market, you earn money when a currency goes up or down with respect to another currency. This trading is always done in currency pairs.
Like EUR/USD. If you buy Euro, then you will earn money if Euro goes up w.r.t. the US Dollar. But if it goes down you will lose money.
It's a very risky place. Start trading with a demo account first and master your basics before jumping on to the real world trading. Almost every forex broker gives you demo account to start with.