The built-in predicate true/0 is always true.

Given the above fact, one can ask:

is socrates a human?

?- human(socrates). Yes

what things are humans?

?- human(X). X = socrates

Clauses with bodies are called rules. An example of a rule is:

mortal(X) :- human(X).

If we add that rule and ask what things are mortals?