hooks are macros (or placeholders) stored in specific locations inside other macros that can be redefined to the user's specifications, thereby "hooking into" the macro at that specific point.
Common hooks used in documents include
\AtEndDocument that adds contents to the hooks
\@enddocumenthook, respectively. The
etoolbox package provides a number of tools to hook into macros (via
\patchcmd, say) and environments, amongst other pre-defined hooks.