A spread is a difference between the current price at which we can buy an asset (ASK) and the price at which we can sell an asset (BID).
For example, the current quote for currency pair EUR/USD is Ask=1,39801, Bid=1,39789. In this case spread is 12 points, since the price differs by 0,00012$. Spread is also a commission that broker takes for opening a position in Forex market. If we’re talking about the market spread, you should consider that it is widening due to some financial news and increasing market activity. It means the faster the market price is changing, the wider a spread will be.