What is altgr in keymap?

[Arnaud Bergeron <abergeron_(_at_)_gmail_(_dot_)_com>]

In my efforts to wrote a canadian-french keymap, I stumbled upon a
problem.  I cannot type characters that are listed under altgr (using
the alt key in combinason with the desired key).

Is the altgr key something else?

Did I miss some very important thing?

I can submit the patch if needed.

Arnaud Bergeron.