We never should have sold those air rights, Ed.
The couple on the bottom apparently sold the air rights above their property, not realizing that whoever they sold the rights two would be able to construct an entire parallel house/property above them. The people on the bottom have a more cramped property now that their vertical neighbors live so close. It's funny in part because of the contrast between the benign/everyday nature of legal concerns vs. the whimsical idea that an upside-down house could be constructed.