The green room in a
theatre is where the
actors relax when they have a break. Rather than have to wait in the
wings, the green room is a room (not necessarily green..) where they can hear what is going on on
stage, and so can know when they next have to be on.
If you're after any other theatre tech information, then do take a look at '
Everything you ever wanted to know about theatre tech, but were afraid to ask'