HSL stands for hue, saturation, and lightness - and represents a cylindrical-coordinate representation of colors.
An HSL color value is specified with the hsl() function: hsl(hue, saturation, lightness)
Hue is a degree on the color wheel (from 0 to 360) - 0 (or 360) is red, 120 is green, 240 is blue. Saturation is a percentage value; 0% means a shade of gray and 100% is the full color. Lightness is also a percentage; 0% is black, 100% is white.