You've compiled your map. It looks great. You submitted it to a playtest, and when the feedback comes back to you, all the complaints are about how bright it is. But how could that be? Well, it's due to a little thing called HDR.
HDR in TF2 is a simulation of how your eye works in real life. When you go from looking at something dark to bright, TF2 "adjusts" the bright scene from the adjusted dark scene, to the bright. You know when you turn on the lights in a dark room, and your eyes close to adjust? It's like that.
HDR has to be controlled, however. The default values for HDR can make some maps incredibly bright to look at, and it makes playing on it a lot more of a hassle. Luckily, with the use of a tonemap_controller and a logic_auto, you can control the minimum, maximum, and level of bloom that occurs from HDR.
This prefab is that. Place it into your map and the pre-set settings will adjust the HDR in your map to be much nicer. No more feedback telling you that player's eyes are being sent into the 4th dimension from walls turning green from how bright they are!