"Endian" refers to the unfortunate fact of modern computing that data that requires multiple bytes to represent is stored differently on different computers. The bytes can be high-order first or low-order first. Binary data stored in the pxgraph format will depend on the computer that generates it. Thus, the pxgraph compatibility features of the plot package permit you to specify the endianness of the data.
The three plots show the same data with different endian arguments. The correct rendition looks like the binary data demo. If you are running on a little-endian machine such as a machine with an Intel x86 processor, then the -binary and -littleendian plots will be incorrect. If you are running on a big-endian machine, such as a Sun SPARC, then the -littleendian plot will be incorrect. See the Ptplot FAQ for further details.