How to read / understand spatial RF charts like "XZ Plane Polarization"
It's a measure of how efficient the antenna is depending on the angle of the transmission/reception.        

The reference is often an ideal isotropic antenna and values given in dBi, but it can be dBm (relative to 1 milliwatt), dBd (relative to a dipole), etc. As it's a 3D object, you need plots for both horizontal and vertical planes (or a 3D plot) to get the "full picture".
Microphones have similar graphs that display the same information. but for sensitivity to the SPL (sound pressure level) Here's a polar plot for a typical cardoid mic: