As far as I can tell, "generalize" is used in its basic meaning. Remember, the Menu block module is capable of creating new (sub)menus and these new (sub)menus will have the same properties (and behavior) as the Main and Secondary menus (but will be lower in the menu hierarchy). "generalizes" refers to the ability of the Menu block module to instantiate as many new (sub)menus as necessary (lower in the menu hierarchy), all having the same properties and behavior as the Main and Secondary menus.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.