Osmosis is the movement of a fluid across a semipermeable membrane from a solution with lower solute concentration to a solution with higher solute concentration. Osmosis continues until equilibrium is achieved.
The above statement, though scientifically precise and accurate, contains many technical terms that should be further elucidated. A membrane is a thin layer of tissue (in plants and animals) or synthetic material (in a machine, or a laboratory setup). A semipermeable membrane is one which allows molecules of certain substances to pass through it, but is an effective seal against any other substance. A basic definition of a solution, for someone without a background in chemistry, would be to describe it as a crystallized or powdered substance (the solute) dissolved in a liquid substance (the solvent). Equilibrium, in this case, is the condition where the solutions on both sides of the membrane have equal concentrations of solute.
Osmosis is a vital process in your body’s endless quest to maintain homeostasis, that is, a consistent and healthy biochemical state. The term osmosis is most commonly used in the special case where the fluid in question is water, and the semipermeable membrane is that of a cell. The cells expend water in the fulfillment of their natural functions, thus increasing the solute concentration inside of them. If you follow a healthy diet, you drink at least eight glasses of water in a day; this water decreases the solute concentration in the cells’ surroundings. Thus, water moves into the cells and allows them to continue fulfilling their duties. This is also why drinking salt water (if you are shipwrecked, etc.) is a very bad idea. The fluid entering your body will have a solute concentration higher or equal to that of your depleted cells, so no water will move into the cells. Dehydrated, they will not function properly, and you will die.