Basically, it's a simple tool that helps web designers find lighter and darker colors based on any color. Why is this necessary? Well, that takes a little more explanation.
I built 0to255 to make my job easier. I'm a web designer who works with colors every day. Usually, I'll start with a color I want to use, then I'll manipulate it for various functions. For example, I might brighten a link in its hover state or darken a button's border to make it contrast with surrounding elements. Web designers do things like these several times an hour.
Most of the time, I'll just guess the hex code (which takes practice and is far from perfect) or, if I'm feeling particularly patient, I'll wait the gazillion years it takes for Photoshop to open. Even then, finding lighter and darker colors in Photoshop is somewhat unintuitive due to it using HSV rather than HSL for its color picker. The whole experience is a royal pain-in-the-arse.
0to255 cures your color manipulation woes. Simply pick the color that you want to start with and 0to255 gives you a range of colors from black to white using an interval optimized for web design. Then, just click the variation you want to use and the hex code is automatically copied to your clipboard.
It's fun, simple, and a huge time-saver.