About `unload-feature'

[Xue Fuqiao]

(info "(elisp) Unloading") says:

Before restoring the previous definitions, `unload-feature' runs `remove-hook' 
to remove functions in the library from certain hooks.  These hooks include 
variables whose names end in `-hook' (or the deprecated suffix `-hooks'), plus 
those listed in `unload-feature-special-hooks', as well as `auto-mode-alist'. 
This is to prevent Emacs from ceasing to function because important hooks refer 
to functions that are no longer defined.

What does `ceasing to function' mean?  Can anybody help?  Thanks.
-- 
Best regards, Xue Fuqiao.
http://www.emacswiki.org/emacs/XueFuqiao