We know that the motion of molecules tends to distribute them equally in any enclosed volume. For example, steam produced from a hot shower can be seen to fill the whole bathroom more or less equally. Consider what happens when someone opens a perfume bottle. The smells are able to diffuse from the bottle, where they are concentrated, to the rest of the room.
Osmosis is merely diffusion of water molecules across thin barriers called membranes.
Such membranes are common in nature. They separate cells from one another, and define boundaries between different organs. In your body, osmosis plays an important role in controlling the flow of molecules between the blood and other tissues. Osmosis maintains the balance between the blood and the other tissues. The water carries nutrients and wastes in opposite directions when it diffuses in and out.