Simmering vs. Boiling
To simmer is to cook something just below the boil. The idea is to cook the food gently without evaporating the liquid.
Boiling is not only designed to cook food in hot liquid, but to keep it moving, and often to reduce the liquids level and concentrate its flavors.
When liquid boils, bubbles rise to the surface and pop, releasing evaporated water (AKA “steam”) into the air. This concentrates the liquids flavor, and reduces the amount of water. It’s the principle behind sauce reductions.
Simmering foods are often covered to help retain as much moisture and flavor in the pot as possible. Boiling foods can’t generally be covered, as the pressure and amount of steam is always stronger than a wimpy lid.