When you’re bullish on an instrument, you want to go long in expectation of the price to appreciate and selling it at a higher price, thus making a profit.
For example, if you buy a stock, then you’re going long that particular stock because you expect the price to increase in the future and it’s said that you are long or have a long position. You’re going long also when you buy a derivative like a CFD, a futures contract or an option.