A rule represents an 'if-then' statement. For instance:
If ?x has a parent ?y and ?y has a brother ?z, then ?x has an uncle ?z.
These rules are expressed using datalog, which has the following format:
In this format, the left-hand side of the :- operator is the rule head (the 'then' part), and the right-hand side is the rule body (the 'if' part).
Essentially, this rule implies that if both conditions, [?x, :hasParent, ?y] and [?y, :hasBrother, ?z], are satisfied, then the condition [?x, :hasUncle, ?z] is also true.