A regular water softener operates in an intermittent fashion -- it runs for a while, then needs to be regenerated. This idea is for a device which can run continuously, for as long as it's supplied with salt.
Osmosis is the movement of water across a semi-permeable membrane from an area of high water
potential (low solute concentration) to an area of low water potential (high solute concentration).
If this membrane is in a counter-current exchange device, we can pump low solute concentration water in on one side, and have it come out with a high solute concentration, provided that in the other section of the counter current exchanger, we pump in high solute concentration water, which come out as low solute concentration water.
Thus...
We take hard tap water, full of calcium and other minerals, but which is nonetheless mostly water, and pass it through our counter current osmotic exchange filter. This fluid comes out of the device at a very high mineral concentration, and is discarded.
Meanwhile, we take highly concentrated salt brine, and pump it into the device... it comes out extremely dilute, and we then use it for drinking, cooking, bathing, etc..