A "reference" system is your stereo system at home home that you reference when you are unsuccessfully trying to hit on that hot blonde at the bar. Seriously, it is a system so resolving and familiar to the owner that he can swap in a single piece of gear and immediately hear the differences in its sound quality from the piece that it replaced.
What is a "reference" system?
OK, most of us have an audio system, mine even works some of the time. But I see that some members have "reference" systems. This has to be better than the assorted junk that I have piled together. Probably a lot better than your rig as well.
But really, what distinguishes these folks "reference" systems from the pedestrian systems that the rest of us have? There must be something, or they could just save the keyboard time and drop the word when discussing their gear. And I am not referring to reviewers, that is a different story and one to examine more carefully in the future.