While watching a movie, in a dialogue scene, Ive heard this character saying "you are a tough room, do you know that?". What is the meaning of tough room? Is it an idiom?
A tough room is an audience. The performer on the stage thinks or says it if the audience does not react the way he or she expected. Or the performer may pretend they're not reacting properly. For a comedian, this means that they're not laughing at his jokes.
"Room" is performers' slang for auditorium. Saying "You're a tough room" outdoors, or in any place that in no way resembles an auditorium, is actually a joke.
The word "tough" here means it's difficult to make this particular audience laugh (or cry, or whatever the performer expects them to do).
The room can also get very tough very soon if, say, the performer starts making jokes about Rembrandt without realizing that the audience consists mostly of art experts who know a lot more about Rembrandt than he or she does.