J2 is a coefficient in a typical model of gravitational potential of a planet (Geopotential Model). It reflects, for example, a planet's oblateness due to rotation, specifically the deviation of its mass from spherical symmetry.
J2 is of interest for space flight navigation and is measured by observing the flight of spacecraft near a mass. Its measure aids in modeling the composition of a planet, thus measuring that of solar system planets is of interest. Cassini has been used for this with Saturn and Juno will be so-used with Jupiter.
It is also useful in modeling the behavior of rings.
J3 is a similar coefficient but reflects asymmetry across the equator, thus is typically far less significant than J2.
Laplace Radius (rL)