What does this equation mean?

Gravity. A planet’s external gravity field is not sensed with a gravimeter sitting on the planet; it is read off the motion of an orbiting or flying-by spacecraft, whose trajectory bends in response to mass distribution beneath it. In practice this means tracking the spacecraft’s radio carrier signal from Earth and measuring its Doppler shift with extreme precision as the spacecraft accelerates and decelerates along its orbit. A denser, non-uniformly distributed interior produces small, characteristic deviations from a simple point-mass orbit, expressed as gravity harmonics ( $J_2$ , $C_{22}$ , and higher terms) layered on top of the dominant 1/$r^2$ term. From those harmonics and the body’s known radius and rotation, one can compute the moment of inertia factor ,