When liquid is maintained at a temperature just below a boil, it is called a simmer. When there are tiny bubbles barely breaking the surface it’s considered a “gentle” or “low” simmer. If the bubbles are larger and moving faster, it’s considered a “rapid” simmer.
Simmering cooks food gently and slowly, perfect for delicate foods such as fish or fibrous root vegetables like potatoes. Keeping potatoes at a simmer allows them to cook more evenly.
When liquid is brought to a boil, the bubbles will be numerous, large, and consistently breaking at the surface. At this point, the liquid will also quickly evaporate.
Because boiling keeps food in motion it helps to cook it quickly and prevent it from sticking to itself which is ideal when cooking pasta. Tender green vegetables also do well with boiling because it helps to maintain their color and flavor. Lastly, because boiling causes speedy evaporation of liquids it’s also the best method to use when trying to reduce sauces.