Facts define pieces of knowledge or data in your expert system. Facts define the world of your expert system, i.e., the people in it, the objects in it, etc. Rules manipulate facts to infer new facts (or new knowledge) and to act. Facts can be asserted into the world or retracted from the world. Facts consist of a relation name and zero or more slots and their associated values. For example, the fact '(person (name Tom) (hair-color brown))' defines a person in the world whose name is Tom and has brown hair color.
Before creating facts, you should define the fact templates, or types, for your facts. Facts may be created before defining fact templates, but then the fact consists only of the relation name and one slot that contains everything else. For example, if you assert '(person tom brown)', this defines a person fact which has a slot that contains the field 'tom brown' (a list of values).
See the Fact definition for the syntax of a fact.