Water always moves from a region of higher water potential to a region of lower concentration of water molecules.
In case, the concentration of solutes in the medium is less than the concentration of solutes in the cell sap, the medium is said to be hypotonic in comparison to the cell sap.
Such a medium will have more number of water molecules relative to the number of water molecules inside the cell.
Thus, the concentration of water molecules in the medium will be higher than the concentration of water molecules of the cell sap, and water will move into the cell along the concentration gradient.