Simmering is a method used to cook food in a liquid in higher temperatures at a slower rate, thus bringing out more flavor but not overcooking. Simmering is beneficial for tough meats and more durable vegetables like carrots or potatoes, and is the best bet when making soups and stocks. A house with a dish simmering on the stove is a house that smells great.
When directions call for simmering, it is usually once the dish has been brought to a boil and then the heat is reduced. By bringing it to a rolling boil, certain chemical reactions within the food and the liquid can take place and the cooking process begins. Bringing the liquid to a boil first also aids in speeding up the cooking process. In some instances, you wouldn't want to slowly bring the liquid to a simmer because the proper temperature needs to be reached and maintained; a food that sits in a temperature danger zone for too long, even while cooking, is an environment conducive to harmful bacterial infestation.
How Simmering Works
Simmering involves a slight bubbling but is not a rolling boil. The small bubbles produced by simmering help move the food around for more even cooking. It also helps prevent the items in the cooking vessel from settling and sticking to the bottom. A still dish is not a simmering dish; turn up the heat a couple of notches if you don't see any bubbles. Slow cookers are prime examples of cooking by simmering -- a slower, longer process that extracts flavors and produces tender meat and vegetables.
When to Simmer
If you are creating a stock, sauce or soup, or are looking to tenderize tough meat or thick veggies, you will use the simmering method. The amount of time you will need to simmer a dish will depend on what you're cooking. Beware that over-simmering can also destroy nutrients and toughen meat, so test your food occasionally to see if it's ready. If creating a stock or broth, simmering is best, as fats will be released by the process and you can then remove these by skimming them off the top of the liquid.
When Not to Simmer
If you are in a hurry, do not attempt a dish that requires simmering. Simmering is a delicate process to slowly extract fats when making stocks, prevent a meat like chicken or fish from becoming overcooked or create a chemical reaction when making sweets like marmalade. It is a process that should not be rushed or meat fibers get broken down, broths are clouded, vegetables are left limp and jams become rubbery. If directions call for boiling, make sure you are boiling and not simmering, or you will not be cooking at the right temperature and that can end up being unsafe.