The slot-missing generic function is a function that is not intended to be invoked by the programmer, but it’s still interesting to the programmer in some contexts.  This generic function is invoked by the Lisp system when an attempt is made to retrieve from an instance the value of a slot that doesn’t exist in the class.

By specializing this function, the programmer can attach particular actions to such invalid access attemps.  The ways to access a slot by name in an object are slot-value (with or without its setf operator), slot-boundp, and slot-makunbound.  The invocation of slot-missing allows the programmer to distinguish between these cases.  One possible use would be to add debugging information or logging when one of these invalid accesses is attempted.