This special form arranges for the body forms to be evaluated in a context in which the given function is the handler for the set of conditions specified by the class and it's subclasses.
That is, if a condition is signalled during the execution of the body forms, and the condition object is a general instance of class, then the function is called with two arguments.
The first argument is the condition object that was signalled, which will be an instance of class.
The second argument is the "next handler" procedure, which is a procedure of no arguments which, when called, will arrange to invoke the next outermost handler.
The next-handler procedure may be used to be more selective in which conditions to handle, by invoking it if the function chooses not to handle the condition.
The function is called in the context of the signalling expression. In particular, the handling context established by this handler-bind and any interior handler-bind's will still be in force. Hence, if a condition which is an instance of class is signalled within the execution of function, then function may be called again (and will be, if an interior handler-bind does not handle the condition).