A group of people who either a) don't play a marching instrument such as oboe or bassoon or b) suck ass at the instrument they do play. The cymbal line is usually made up of the rejects and losers who can't play a real instrument and march at the same time.
Student 1: I'm so happy because I got a spot on the clarinet hornline!! How did you do?
Student 2: Well, my audition didn't go so well so the directors offered to give me a spot on the cymbal line!
Student 1: .....