What's a "work tree"?

I'm trying to figure out what a "work tree" is.  as in --work-tree. 
This is a new command right, the tutorials I've read don't have it. The 
man page has the syntax but I don't know what it's for.

$ cd /www/mysitedocroot
$ git --git-dir /gitdir/mysitegit/ add .
fatal: add must be run in a work tree

thanks