Simply, stay quietly. Or don't get mad.

To be "low key" means to be discreet in your activity or inconspicuous in your behaviour. It could be used to describe an actor in a movie who plays a role that is not of great importance. He has a "low key" part, one that goes unnoticed. It could be used to describe an employee who does his job quietly without expecting credit. They do their job without being noticed. "Low key" would be the opposite of "flashy", "showy" and energetic. Hope this helps