A short and simple definition of Emile Durkheim's concept of "social facts" relates to social structures and norms which regulate individual and collective life.
Durkheim thought that sociology should be "the science of social facts". Social facts for Durkheim are elements in daily life which are purely social or cultural and they are characterized by being common to all or at least many members of society. Social facts do not depend on any particular individual nor any natural state of affairs, but from a social point of view they are in fact: facts!.
Although they are not natural, social facts are objective. It is not up to anyone to decide if a certain social norm or institute exists. Like it or not, these are the external factors that together with natural facts shape our existence.
An example of a social fact can be the institute of the family and kinship, religion and language are also social facts that precede us in shaping our reality. Even manners of fashion can be regarded as social facts, to be studied under Durkheim's sociology.
Durkheim's theory was developed in his famous What is Social Fact?. Here you can read a little bit more about the thought of Emile Durkheim.
Marcel Mauss later developed Durkheim's idea and talked about "total social facts". For Mauss a total social fact is an action connected to all or most realms of social existence (like economics, culture, politics etc.)