[Gimp-user] Foreground selection tool "Draw Unknown" meaning

[Val <forums gimpusers com>]

What does the "draw unknown" option actually do? I just edited out A head of a
pop star. I don't know how "draw unknown" helps.
So far I've assumed the unknown area is where the hair is so thin, it's hard to
seperate it from the background.

Amd I doing it right? 

Also, what does "Levels" and "Active Levels" do? I've played with it, but I
don't know how it's defined, therefore I cannot predict what it will do for  me.

Cheers!

-- 
Val (via www.gimpusers.com/forums)