Auto brightness is an option that allows the phone's sensors to detect ambient light, and then adjust the screen brightness accordingly. I can't think of a source off the top of my head, but it has been shown to cause wakelocks, which the drain the battery.. It doesn't override what you have set as the brightness on the slider, the brightest it will get is however high you set it. It's all a matter of personal opinion, but when I'm walking around my house and texting or whatever, I'd prefer it if my screen didn't strobe every four steps..