A long is when a trader buys an asset with the hope of selling it at a later time for a higher price. This difference in price is the profit the trader makes when selling the assets. If Susie buys one XCoin for £5 and sells it for £10 a few hours later, her long trade has returned £5 profit.
Long orders are placed when traders feel the market is bullish and that prices will rise. When a trader decides to sell it is based on their trading strategy, which is determined by the returns they are trying to earn.
Longs are the opposite of shorts, which traders use when they think prices will fall.