HSLa stands for Hue, Saturation, Lightness, and Alpha Opacity. Alpha Opacity determines how opaque or transparent the color is.
The first value is the hue degree (0-360), the second value is the saturation percentage (0-100%), the third value is the lightness percentage (0-100%), and the last value is the alpha (between 0 and 1).
Since the value of alpha is only between 0-1 you can or should use decimals
For Example:
hsla(0, 100%, 50%,1) is cyan, fully saturated, normal lightness, with no transparency because 1 is to make it one solid color
hsla(120, 100%, 50%, .5) is green, fully saturated, normal lightness, with partial transparency
hsla(240, 100%0, 50%, .3) is blue, fully saturated, normal lightness, and almost fully transparent
hsla(0, 100%, 20%,1) is fully opaque (not transparent) and the lightness percentage is at 20% which makes the color darker
hsla(0, 100%, 50%,.5) is 50% transparent since its at .5 and that is 1/2 of 1. So there will be color but it won't be very pigmented
hsla(0, 100%, 50%,0) is fully transparent meaning there is no color and it is see through.