Open system vs closed system

Can someone elaborate on the differences between a closed system and an open system?

In what context?

The following from Renal solutions site is an example of a closed system:

With sorbent dialysis. Nothing during a treatment, not even ultrafiltrate, goes down a drain. Thus, any increase in the volume of dialysate in the system is due to the accumulation of ultrafiltrate unless fluid is deliberately added. Therefore, a direct measurement of ultrafiltration is provided not an estimate. It is not necessary to connect the sorbent system to a drain or to continuously provide water and concentrate to the system during dialysis. As a result, there are no hoses to leak water or to create a hazard.

Sorbent dialysis is a simpler hemodialysis therapy option when compared with far more complex single pass dialysis. This unique sorbent dialysis therapy brings new levels of convenience to hemodialysis.