The ramp weight is exactly that, the maximum permissible weight on the ramp, before takeoff. It often needs to be less for takeoff, and the difference allows for start, runup, taxi, etc so that the pilot can start the takeoff at the maximum permissible takeoff weight. This generally applies to larger aircraft, but the legal terms still apply to all aircraft. The maximum ramp weight may or may not be different from the maximum takeoff weight, and the maximum landing weight may or may not be less than the maximlum takeoff weight.
As to why the ramp is so named, I have no idea. Why is a tree called a tree? Everything needs a name, no?