(defmethod foo ((p person) (s string))
(list p s))
Defmethod was defining an implementation of the generic function foo for arguments person with string.
One could later put something like:
(defmethod foo ((p person) (p person))
(send-greeting p p))
CLOS allows specializing not just on classes, but also on specific instances, so some method applies only for a specific instance. For example a "foo" that applies only to a particular "important" person.