A substitute good is a good that can be used in place of another. It is a good with a positive cross elasticity of demand.
This means a good's demand is increased when the price of another good is increased; both in the same direction.
For example, if apples and oranges are substitutes for a consumer, then if the price of apples increases, the consumer will buy less of apples and more of oranges. Thus, when price of apples increases, the demand for oranges will rise.