### Disclamer

I couldn't find a resource providing a good and for none-Emacs-experienced-users understandable explanation about *prefix keys*. So I am a newbie and my explanation may not be the best.

### Prefix key concept
Emacs has the concept of [prefix keys](https://www.gnu.org/software/emacs/manual/html_node/elisp/Prefix-Keys.html). With them you can modify the usual/default behaviour of a key-binding or its function.

The `C-u C-c C-l` can be described as `C-u` (the prefix-key) **plus** `C-c C-l` (the original key-binding). That is why you aren't able to ask for the keybinding with `C-h k C-u C-c C-l` (`describe-key RET C-u C-c C-l`). Do it **without** the prefix key (`C-h C-c C-l`) and you find out that key-binding points do `org-insert-link`.

In the description of that function the possible prefix-keys and what do they modify in the function's behaviour are described. For `org-insert-link` you can also use the prefix-keys twice: `C-u C-u C-c C-l` which cause treating the file path as absolute instead of relative.