It is the tendancy for the back end to shake differently from the front end. Usually, drivers/passengers notice this by watching the windscreen surround moving differently than they expect. It can also manifest itself by watching the dash in relation to the windscreen or doors, etc.
When you go over a particularly bumpy road, the windscreen will rattle side to side; some people are really bothered by this, and others are not. The fixed roof cars experience this too, but much much less visibly. Usually the fixed roof tightens things up a whole lot. Of course, in convertibles, other parts of the car are stiffened in order to try and make the car solid.
I used to drive (way way way back) a 1975 Alfa Romeo Spider convertible. I was constantly amazed that the windscreen remained attached to the car. It was a GREAT car to drive and I loved it, however, it shook and rattled more than any other car i've experienced. In comparison, the EOS is a solid piece of stone that is amazingly solid.
In a perfect world, the car itself would remain solid and the shocks and tires would handle any road bumps. That isn't possible, however, and the metals and structures within the car end up taking some of that by flexing and twisting a little. A little makes for a slightly smoother ride, but alot of flexing can be unsettling. Plus, the frame flexing that we are talking about here can affect handling and performance.