All I know that it's not a percentage, so how does it work?
What does the value represent?

It's a rough indicator for the likelihood that any given block, within the bounds of a vein, will be an ore block. Since most of the veins have non-uniform internal densities when calculated (e.g. spheres have a radially decreasing density unless the uniform option is specified), the density isn't as simple as just a percentage. If you really want the calculation you're welcome to look at the various implementations of VeinType#getChanceToGenerate.

Short answer, higher values = more dense veins, linearly scaled. Veins have nonuniform density so experimentation is required to obtain desired results.

Thank you!