Osmosis is defined as the movement of solvent molecules through a semi-permeable membrane. The solvent molecules (often water) will naturally move through the membrane from areas of high concentration to areas of low concentration. This has the effect of equalizing the concentrations of two different solutions separated by a semi-permeable membrane. This type of membrane allows the water molecules to pass freely through it but the solute molecules (the substances dissolved in the water) cannot pass through and as such are trapped on their respective sides of the membrane.
An example of osmosis is putting a slice of tomato into a solution of saltwater. The fiber of the tomato acts as a semi-permeable membrane allowing the water to pass through. The saltwater has a lower concentration of water due to the presence of the salt molecules compared to the water inside of the tomato slice. As a result, the water will transport out of the tomato slice over time and into the surrounding saltwater solution, thus making the tomato slice shrivel up and become smaller.