Osmosis is the scientific process of transferring fluid between molecules. When molecules move in and out of a cell to achieve the same concentration of something, like salt, on both sides, then osmosis is happening.
Osmos means "thrusting or pushing," and the scientific process of osmosis happens when fluid in a super-concentrated area thrusts through little holes in the cell's wall and heads for a less concentrated place. The fluid might go back and forth a few times until both solutions are equally concentrated. We also use osmosis outside of science to mean "subtle, gradual absorption of information." Fall asleep to enough Western movies, and by osmosis, you will eventually become a real cowboy.