I don't quite understand what you mean, could you try to explain a bit better?
I'll do my best.
The goal of a level designer is to create levels that emulate the environment set by the game. If the game you are designing for has a lot of rounded corners and ups and downs, it's going to be really weird if you make a map that is kind of flat and square corners. The square corner map might play really well, but it's not a good map for the game because it doesn't fit the environment of the rounded-circle/hilly map.
For TF2, you're emulating realistic environments. Things that we could see almost picture in the real world. I'm sure you've walked around somewhere and have gone "Huh, this would look really cool in TF2" (Just me? nevermind :c) ... thats the environment part of mapping.
Maybe that helps?