Before you are able to understand osmosis, you must learn a few other things first. We all know what cells are, around a cell is the cell membrane which helps organisms keep a stable level of internal conditions, homeostasis. When substances pass through the membrane without getting energy from the cell; it is known as passive transport. Think of a sugar cube, when you put the sugar cub in water it sinks to the bottom. The concentration of the sugar molecules is greater at the bottom of the cup then the top. When the cube dissolves it diffuses and sinks to the bottom. In this example the solute was the sugar and the solvent as the water. It is also possible for the solvent molecules to diffuse in a process inwhich water molecules diffuse across the cell membrane from an area to higher concentration to an area of lower concentration. This is called osmosis, which is a type of passive transport becuase when the water moves down it doesn't require energy.
external image osmosis3.gif

In this diagram it shows how osmosis works by crossing the cell membrane going from low concentration to high concentration.

By the end of this presentantion if you still do not understand osmosis please press this link, it will take you to a highly advanced animation with colorful pictures to explain what osmosis really is :)