The spread is the difference between the ASK price and the BID price. The BID price is the rate at which you can sell a currency pair, and the ASK price the rate at which you can buy a currency pair.
Example:
You are trading EURUSD:
Bid Price: 1.18144 / Ask price: 1.18161; hence the spread will be 1.7 pips or 17 points. You can see this on your market watch on the MT4.