Functionalism is one of the major theoretical perspectives in sociology. Its roots go back to Emile Durkheim, whose work forms the foundation of the theory. Functionalism sees all social institutions as organs of a body. The society is the body and its institutions work to keep it alive and well.
To functionalists, all major aspects of a society have an important role in keeping the society stable. The family, for example, provides an outlet for sexual impulses and a way of bringing children into the world and socializing them. Schools further the work of socialization and also help to ensure that children learn what they need to learn in order to be economically useful members of society.
In these ways, social institutions act (in the functionalist view) to keep society alive and stable.