Osmosis is the movement of water from a dilute solution (e.g. with a high water to solute ratio) to a more concentrated solution (e.g. with a low water to solute ratio) through a partially permeable membrane. The membrane allows the water molecules through but not the solute molecules because they are too large.
The effect of osmosis is to gradually dilute the solution. This is what happens at root hair cells, where water moves from the soil into the cell by osmosis, along a concentration gradient.