When you're hot-boxing a room/car/anything you can hotbox while smoking pretty much anything, usually marijuana, and somebody opens the door or cracks a window. They "break the seal" on the hotbox and let the smoke out.
Smoking Newbie - "Dude, I'm getting so high, I need to get out of here." He reaches to open the door.
Stoner - "Fuck No! Don't break the seal!"
Smoking Newbie - "I think I'm going to pass out..."
Stoner - "As long as the seal isn't broken we'll be alright."