A supersaturated solution is a solution with more dissolved solute than the solvent would normally dissolve in its current conditions. Supersaturation is achieved by dissolving a solute in one set of conditions, then transferring it to other conditions without triggering any release of the solute. Supersaturated solutions are extremely unstable, but often require a triggering event to begin returning to a stable state via the solute coming out of solution.
A common example of a supersaturated solution is the carbonated beverage. These have much larger amounts of carbon dioxide dissolved than would be possible in normal conditions. The gas is kept dissolved by increased pressure, but immediately begins forming bubbles of released gas where the solution is in contact with its container once that pressure is released through opening the container.
Supersaturated solutions of common substances such as sugar are also possible. Water can dissolve more sugar at higher temperatures, so cooling a carefully prepared solution of fully concentrated sugar water from high temperatures results in a supersaturated solution of sugar. A string or other object placed in the solution gives the sugar crystals a place to come out of solution, and any object thus inserted slowly develops a coating of solid sugar. Faster and more dramatic reactions are possible with different solutes.