What does out of square mean with respect to my windshield?

It means that the windshield opening (cap) is no longer true to its original production shape. Over time a coach can shift from side to side or twist from front to back in such a way as to cause temporary or permanent deformity. Think about the inherent movement of a large RV when turning into a sloped driveway. As this happens repeatedly over time, or perhaps due to a single trauma event, the windshield opening in some coaches can lose their shape. While it may not be easily visible to the naked eye, it can make a significant difference in the fit of your windshield. Pop-outs and stress cracks are a sign of this problem. Here is more info