Retrogradation occurs when starch molecules that dispersed during gelatinization begin to reassociate as food cools and is stored. It contributes to the firming of cooked starches and the staling of bread.
It is not simply drying: bread can firm even in a closed package. Reheating can temporarily soften some foods, but it does not reverse every storage change or make improperly stored food safe. Water, temperature and the kind of starch affect the process.
Reference: Further reading.