In the three-dimensional world, Homer encounters a signpost indicating the directions of X, Y and Z, with X and Z indicating right and back, with Y pointing upward. But X and Y are the directions of the flat plane, which in three dimensions is considered to be the floor. The "up" direction, that gives 3D objects their solidity, is normally labeled Z.
The aliens, Kang and Kodos, are shown hitchhiking. They sport a destination sign reading "EARTH CAPITAL". It *could* be attributed to English presumably being their second language, but the word they wanted was "CAPITOL".