f.lux makes the color of your computer's display adapt to the time of day, warm at night and like sunlight during the day.
Ever notice how people texting at night have that eerie blue glow? Or wake up ready to write down the Next Great Idea, and get blinded by your computer screen? During the day, computer screens look good - they're designed to look like the sun. But, at 9 p.m., 10 p.m., or 3 a.m., you probably shouldn't be looking at the sun. It's even possible that you're staying up too late because of your computer. You could use f.lux because it makes you sleep better, or you could just use it just because it makes your computer look better.
Tell f.lux what kind of lighting you have, and where you live. Then forget about it. f.lux will do the rest, automatically.