Crystallization refers to the formation of sugar crystals in a sugar syrup. Crystallization can occur with the presence of a single unincorporated sugar crystal, and the resulting change in texture—from smooth and fine to lumpy and grainy—is unpleasant and undesirable in many candies. Stirring sugar syrups while boiling encourages crystallization, which is why many recipes advise you not to stir until the proper temperature is reached.
Other methods of avoiding crystallization include putting a lid on the saucepan while boiling a syrup (the steam forms condensation that runs down the sides of the pan, preventing crystals from forming on the pan walls) or washing down the sides of the saucepan with a pastry brush dipped in water. Crystallization can also be prevented by adding an interfering agent.