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