The phase shift indicates how out-of-step the phases of horizontally-polarized and vertically-polarized light became by passing through the part. This becomes important if the light was initially polarized, or subsequently passes through another optical element which further polarizes the light. If so, the transmitted light may vary in brightness or exhibit colored bands.
The meaning of horizontal and vertical depends on the orientation of the light source. They are mutually orthogonal planes of polarization, the intersection of which contains the incident light beam, for each location on the part. The value displayed is for the planes which maximize the phase shift, i.e., the worst case.
This result is meaningful only on the sides of the part not facing the light source. The result is absent for sides of the part facing the light source because there has been no phase shift at this point.