F.lux is based on a very, very simple idea. It comes from the basic idea that "white" is not a universal when it comes to human vision, because not all light sources are the same. That's why "white balancing" photographs is necessary, because you need to balance colors appropriately for the given light source. But our eyes do this automatically. If you take a white sheet of paper outdoors, indoors, wherever, it will generally always appear white to your eyes, even though its appearance will change color in an absolute sense.
This is where computer displays confound things, because they are a source of light, but they are used among other sources of light. And those light sources change over the course of a typical day. And thus the white balance changes over a typical day. Let's say you're using a computer in an office with white walls. Those walls don't seem to change color throughout the day, but they very much do, because the light illuminating them changes. From sunlight to artificial light. Your eyes adjust to this naturally though. But computer displays don't change, which means that during daylight hours your eyes might perceive the display as being normal while after sunset your eyes start to see the display as blue-ish or excessively bright. Alternately, your eyes could perceive your surroundings as different, seeing artificial lighting as yellowish and dim.
F.lux is simply an attempt to harmonize the white balance of your display with the white balance of your surroundings, based on some information about the type of lighting you have and the local times of sunrise/sunset. Once you get used to it it seems perfectly normal that displays should do this.
The problem is that a lot of the underlying technology (monitors, display drivers, OSes, etc.) are built on entirely different assumptions, the idea that the display itself is a universal arbiter of the "correct" color choices and there's no need to white balance harmonize with the rest of the world. Which means that sometimes it's difficult to get something as simple as F.lux to work on a given platform.
That's why it's a topic of frequent conversation. It should really be a feature built into all displays and operating systems as it's a very straightforward idea. And essentially every digital imager in the world is built based on a universal understanding of the need to account for white balance, but there is still an uphill battle in getting people to understand the need for white balance management in digital displays.