The term 'Foreign Exchange' (Forex) means buying or selling one currency using another.
As currencies are traded in pairs, you need to speculate which one will strengthen, buy it and sell the other.
For example: if you believe that the Euro (EUR) is going to strengthen against the dollar (USD), you will buy the EURUSD; or in other words, buy the EUR and sell the USD.
Alternatively, if you believe that the EUR is going to depreciate against the USD, you will sell the EURUSD; or sell the EUR and buy the USD. There is no need to wait for a bullish market to profit because currencies are very volatile and are constantly moving.
The FX market is therefore, constantly producing new opportunities to invest.