Spread is defined as the amount between the bidding price and the asking price. For example, the EUR/USD bid/ask currency rates at your bank may be 1.3025/1.3045. This represents a spread of 20 pips.
There are 2 different kinds of spread: floating and fixed. Fixed spread is always constant so the broker gives the same spread on the given instrument at any time. Floating spread continuously changes as supply and demand change.