If you double click them and look at the properties in the Script tab of the reference window, you'll find out. They attach effects to the camera. (Duh) Things like mist, water dripping from the ceiling, leaves flying past, and even snow. It's for if you want to have those kinds of effects in damp rooms, wooded forests, etc. Go outside, anywhere where you see leaves (or pine needles in pine forest) floating about passed your camera, you're inside one of those triggers. It can really improve interiors and I certainly recommend the use of them!