High Quality Bloom is a graphics setting in Experimental Rust that affects how good the "Bloom" in the game looks. So what is Bloom?
Bloom is a graphic setting that refers to the effect of natural light, swaying/moving trees, plants, flowers, and etc. It makes the world seem more "alive", as players call it. High Quality Bloom is just Bloom tweaked to a level that seems more realistic. You can toggle High Quality Bloom on or off in the F2 settings overlay in-game. For optimal FPS, you should turn it off. High Quality Bloom is on by default.