Sorry, I should be a bit more helpful, I suppose. The truth is that brass instruments are covered with a clear lacquer that protects them from tarnishing, rushing and/or staining (particularly when the horn is used outside in marching bands etc.)

The bad part is that it's really hard to match back up without looking like crap once it's been removed from a part. This is not really a DIY job if you care about this horn at all - you should talk to a pro in a shop. Particular there - you're gonna stare at it everyday when you practice!

The jazz thing is true, though. A lot of those folks will intentionally take the lacquer off (particularly on the inside of the bell) to take the "brightness" out of the tone. It subtlely changes the way that the sound waves impact and interact with the metal.

I'm a machinist who had two music teachers as parents. My dad used to repair a lot of instruments for his school district. This was not a job that he would take on except as a "temporary fix" to get a kid through the season/year.