What does GPS satellite constellation mean?

The constellation of GPS satellites consists of 2/kloc-0 working satellites and 3 in-orbit standby satellites, which is called (2 1+3) GPS constellation. Twenty-four satellites are evenly distributed on six orbital planes, with an orbital inclination of 55 degrees and the distance between orbital planes of 60 degrees, that is, the right ascension of the intersection points of the orbits is 60 degrees apart. The elevation distance between the satellites in each orbital plane is 90, and the satellites in the same orbital plane are 30 ahead of the corresponding satellites in the adjacent orbital planes on both sides.