Explain it like I'm five version : a plumb bob at the Greenwich observatory doesn't point at the center of mass of the earth. This is because of different densities of rocks below the surface, and of mountains on the surface, and so on. Prior to the satellite era, the meridian plane was defined to be parallel to local surface gravity at Greenwich (so the plumb bob string lies in the meridian plane) and perpendicular to the rotation of the earth as determined by observations of stars. When satellites started to be used, the new meridian plane was kept parallel to the original but moved over to go through the center of mass of the earth. The center of mass of the earth is a nice reference point for satellites orbiting the planet. The new system chose to keep noon (the time when the sun is directly overhead, as measured by local gravity direction) at Greenwich fixed instead of keeping the location of the meridian line fixed. That was because lots of people depended on the Greenwich Mean Time standard and not so many people depended on the exact longitudinal coordinates of any location, which prior to GPS had been very hard to measure anyway.