Ride height is how high the chassis sits at rest, measured from a level floor to a fixed point on the frame at each corner. You set it with spring collars or jack bolts, and changing it moves cross weight.
Measure to the frame, not the body. Speedway Motors is specific about it: measure “from the floor to the frame to eliminate any variation in body panels.” Pick four spots, mark them, and use the same four forever — the absolute number matters less than the fact that it’s repeatable. Ride height and cross weight are joined at the hip, because every turn of a jack bolt moves both, which is why Grassroots Motorsports tells you to “always record the cross-weights and ride heights for reference at the race track.” Check your rulebook too; many classes tech a minimum height. RC racers set ride height the same way, off a flat setup board with a gauge, and set it before you touch anything else.
Ride height sets where every other suspension part starts its travel, so an unrecorded change quietly rewrites your whole setup.