A supersaturated solution is a solution that contains more solute than could be dissolved in the solvent at a given temperature. It is unstable because the excess solute will tend to come out of solution, either through precipitation or by forming crystals. This can happen if the solution is disturbed or if the temperature changes, as both of these factors can cause the solute to come out of solution.
Supersaturated solutions can be made with any solute and solvent, not just sodium chloride. To make a supersaturated solution, you would first dissolve as much solute as possible in the solvent at a given temperature. Then, you would increase the temperature of the solution, which would allow the solute to dissolve more completely. Finally, you would cool the solution back down to the original temperature, which would cause the excess solute to come out of solution.
Supersaturated solutions can hold more solute than a saturated solution, which is a solution that contains the maximum amount of solute that can be dissolved at a given temperature. However, they are not stable and will eventually come to equilibrium, with some of the excess solute coming out of solution.