The shoulder slope measurement is an indicator for the amount with which your shoulder slopes downward.
To measure your shoulder slope, you need to determine the angle at which your shoulder line slopes downward.
A good way to measure your shoulder slope is to take a a selfie and rotate it until your shoulder line is horizontal.
The angle you had to rotate is your shoulder slope.
Almost all people have a slightly different shoulder slope on both sides. That’s because your dominant arm is more pronounced.
Unless the different is huge, best not to worry about it. It won’t show up for casual garments, and for formal wear, you’re better to add assymetric shoulder padding, rather than create an asymetric pattern.