A Jumping Roundhouse Kick (also called a Jumping Turning Kick) is a variation of a Roundhouse Kick. Normally it is performed by first thrusting the non-kicking knee high into the air and then jumping upward with the kicking leg, then perform a Roundhouse Kick while in the air, landing on the non-kicking leg.
This is similar to a Jumping Front Kick in that the kicking leg is also the leg that propels you into the air. The non-kicking leg is first thrust knee-high into the air to give your body extra upward momentum at the moment of the jump, to increase the height of your jump. This is different however from a Jumping Side Kick for example, where the non-kicking leg is the one that does the jumping.