A colligative property is a property (of a solution) that can be changed by adding more of a solute. What makes colligative properties different is that the identity of the solute doesn't matter, only the concentration of the solution. For example, if you add a certain amount of a salt to a certain amount of water, the boiling point of the water will increase and the freezing point will decrease. This is the basis for such practices as putting salt on roads so that ice doesn't form.