Your direction reflects whether you expect the price to rise or fall:
Long (Buy): you expect the price to rise. The position opens at the Ask price, and you profit if the price goes up.
Short (Sell): you expect the price to fall. The position opens at the Bid price, and you profit if the price goes down.
Because CFDs don't require owning the asset, you can aim to profit in both directions.
Example: if EUR/USD rises from 1.1000 to 1.1200, a buyer (long) profits; if it falls from 1.1000 to 1.0800, a seller (short) profits.
See the worked example in Trading basics.