Dental bridge margins: what are they?

I have the second replacement of a five tooth bridge on my bottom teeth. The first one broke after just two weeks. The second one feels to high. The dentist said the margins were off. He fiddled with it four or five times and then drilled porcelain off my top two caps twice (which I thought was a weird thing to do). it feels higher than my other side and at normal rest the teeth on the other side don't touch each other. The dentist refused to send it back to the lab. I do not understand what margins are.