HSL is short for Hue, Saturation and Luminance (or brightness).
The HSL color space defines colors more naturally: Hue specifies the base color, the other two values then let you specify the saturation of that color and how bright the color should be.
After specifying the color using the hue value you can specify the saturation of your color. In the HSL color wheel the saturation specifies the distance from the middle of the wheel. So a saturation value of 0 (0%) means "center of the wheel", i.e. a grey value, whereas a saturation value of 1 (100%) means "at the border of the wheel", where the color is fully saturated.
The third parameter in the HSL color space is the luminance. This lets you specify how "bright" the color should be. 0% means,
the brightness is 0, and the color is black. 100% means maximum brightness, and the color is white.
The following six luminance sliders show you what happens with colors if you change the luminance: