The HSL color model stands for Hue, Saturation, and Lightness:

• Hue defines the color position on a color spectrum, and its value specifies the degree from 0 to 360.
• Saturation defines the color intensity and how bright or pale the color looks. It's a percentage value, where 100% means the full color, and 0%, on the contrary, represents the grayish color.
• Lightness shows in percentage how much black or white mixed with the base color. 0% is dark (black), and 100% is white.

Like in RGBA colors, HSLA uses "A" for opacity with the highest value of 1.0. (opaque color) and the lowest — 0.0. (fully transparent color).