The everypage package speaks of hooks.
What exactly is meant by this and how could it be used?
Programmers call "hooks" pieces of code that are executed every time something happens, and that can be "added to" by other programmers. This is like a hook you may have on your bag: you can put your phone or your camera on it. What exactly you hook there can be decided later; the hook is there for anything.
For example, you may print on every page a draft mark. Or send yourself an e-mail "Page 2 is printed". You add the program that does this to a page hook, and it is done every time you ship out a page.