Swing Trading-Sell
In Swing Trading, as with all other kinds of trading the aim is to make profits.
When a trader goes short, the aim is to buy the sold currency at a lower rate than what it was sold at.
If GBP/USD was traded and the trader went short then it means they sold GBP and bought USD. At the time of closing the trade, if the exchange rate falls they will be able to buy back the GBP using less USD than they got for them at the time of selling. That difference is their profit.