Soap is a substance used for cleaning and washing objects or people. It is water-solubile but also dissolves and loosens fat (and oil) molecules that are stuck to a surface, which is where the cleaning properties of soap combined with water come from.
Soap has a bad taste. This is one of the reasons why mouthsoaping is used as a punishment. Soap also irritates the eye and is very painful if it accidentally gets in your eye, which can easily happen when shampooing.