A hypertonic solution is water that has a lot of salts dissolved it. A hypertonic solution has what you call a high "osmostic pressure". If you put it a cell in a hypertonic solution, for example, the water in the cell realizes that the surrounding environment has more salts, and the water will actually leave the cell to balance out the "pressure". This is the only choice, since the salts can not go into the cell due to the membrane, and the leaving of water causes the cell to shrink.
Now to get to how hypertonic solutions might be used in medicine. Hypertonic solutions are used as a treatment for people who have cerebral edemas. Cerebral edema is an accumulation of water in the brain, which causes swelling that is not good for the brain. The hypertonic solution helps that extra water leave the brain cells. I did a bit of research and did not find anything about hypertonic solution being used on its own to treat cancer, although I did find some research studies where drugs and hypertonic or hypotonic solutions were delivered together. That said, research studies do not mean that it is a treatment being used in the clinic today. In fact, a list of standard cancer treatmentsclick here (if you have access to internet) does not show hypertonic solutions to be a standard treatment.
My hypothesis about this is that maybe it is not effective enough on its own to kill cancer cells. Another possibility is that the solution could be difficult to deliver to the patients; you would have to know how much to give and make sure it gets to the cancer cells rather than effecting healthy cells. Excellent question!