Every forex quote is two prices: the bid is what you can sell at (the market will buy from you at this price), and the ask is what you can buy at (the market will sell to you at this price). The ask is always slightly higher than the bid, and the gap between them is the spread.
When you open a long position you pay the ask price; when you close it you sell at the bid price. That round trip costs you the spread before the position even starts moving — one reason low-spread brokers and ECN accounts matter for high-frequency and scalping strategies.