The second part of a dance called the "IBM Torture Boogie". Designed simply to harm cool people.
The acronym stands for System Network Architecture and was orignally designed to work with IBM's other old networking standard, Twinax. It was one of the first attepts at real workstation-to-server interaction beyond the terminal emulation level, though due to infrastructure and implimentation overhead, it failed miserably.
For a while, Microsoft marketed the "Microsoft SNA Server" for those wishing to migrate, but it has been moved out of focus as of late.