I think I had trouble digesting it a few months ago because it depended on so many utilities I was unfamiliar with: vtables, defmethod, pickles (and it compared with extend, which I didn't understand back then :-o ). Giving it another try...
vtables and pickles aren't utilities, just implementation details for defgeneric.
Basically vtables contains a hashtable for each generic function mapping a type to an implementation. "If len gets a string, do this. If it gets a table, do that." The body given to defgeneric sets up vtable entries for a few default types (cons, mainly :), and defmethod lets you add to vtables later.
If the generic function doesn't find an entry in vtables it falls back on searching the pickles table for a procedure to convert that type to cons, before retrying.
Let me know if this makes sense.
(names: I believe vtables comes from C++, and pickle is the python primitive for serialization)