The prefilled cartridges consist of a black plastic tube, polyfill, and juice (generic Marlboro flavor). If you take out the polyfill and rinse the juice out of the cartridge you have a blank cartridge. If you rinse the polyfill, pat it dry, add some juice, stuff the polyfill back in, and top it off with more juice, you have a refilled cartridge.
It's a lot to get straight when you first get a kit. I remember looking at the darned thing and thinking that the instructions weren't nearly good enough.