For example if the ionic dissociation is complete, NaCl produces two ions in solution, BaCl2 produces three ions in solution, etc. However, the dissociation is not quite complete so we will not generally have these nice integers for the number of particles in solution. Some values of the van't Hoff factor are given in Table 14.4 on page 666 as a function of concentration for two ionic compounds. You see that as the solution becomes more dilute, the van't Hoff factor approaches the expected integer. The final colligative property is the osmotic pressure.
Osmosis is the movement of solvent molecules through a semipermeable membrane.
The spontaneous direction of osmosis is from the region of low solute concentration to the region of high solute concentration. Considering the prune in the water, we see that osmosis of the water will occur from the region of low solute concentration to the region of high solute concentration. The osmosis will continue until the prune skin bursts from the increased pressure inside the prune - we try to stop the process before we have a mess. This increased pressure is called osmotic pressure because the cause of the increased pressure is the osmosis. It is always very important to understand what is the cause and what is the effect in any process.