Bid/Ask Spread.

A spread is the difference between the bid and the ask price. The bid price is the price at which you may sell your currency pair for. The ask price is the price at which you must buy the currency pair. The ask price is always higher then the bid price. Profits in the market are made from charging the ask price for a currency pair and buying it from someone else at the bid price.
The bid/ask spread increases when there is uncertainty about what is going to happen in the market.