I was hoping somebody else would jump in, but what the heck. BYTESWAP8, to the best of my knowledge, does nothing in standard C because it isn't defined there. Searching for it with Google leads me to suspect it is commonly a macro (preprocessor pseudo-function) that reverses the order of the bytes in an eight-byte word.
The reason you'd want to do this is if you have a binary data interchange format (file or network packet) that needs to interoperate between big-endian and little-endian machines. http://en.wikipedia.org/wiki/Endianness
