A currency is the official money of a country, region, or monetary authority. In the forex market, a currency is the instrument you buy and sell.
Currencies use three-letter ISO codes: USD for the US dollar, EUR for the euro, JPY for the Japanese yen, and GBP for the British pound. The codes keep forex quotes readable across global markets.
Currencies trade in pairs, never alone. The first is the base currency, the second is the quote currency, and the exchange rate shows how much of the quote it takes to buy one unit of the base.
You look at GBP/USD = 1.3740, where GBP is the base currency and USD is the quote currency, so one British pound equals 1.3740 US dollars.
Expecting the pound to gain on the dollar, you buy GBP/USD. Expecting it to weaken, you sell GBP/USD.