RHCP or LHCP? what does it mean? does it mater? which is best?

RHCP and LHCP stand for Right Hand Circular Polarization and Left Hand Circular Polarization. Circular radio waves are a bit more than circular, they are spiral, like a screw thread. That “thread” can then turn one way or the other. Right hand screws are most common, but a left hand screw is just as strong and will do the same job, same for antennas. You won’t see a difference if you run a set of LHCP antennas vs RHCP, it’s a matter of choice.  The important thing to remember is to match antennas polarization on receiver and transmitter. While it is impossible for a left hand screw to into a right hand thread, the LHCP antenna will still work with a RHCP counterpart, but the results will be inconsistent and range much reduced.

Note about the DJI air unit and CaddxFPV vista: They comes equipped with somewhat LHCP antenna(s), so if you purchase antennas for these systems, it make sense to maintain compatibility by selecting LHCP antennas.