When cooking, it is generally recommended to simmer with the lid on to retain heat and moisture, helping the food cook evenly and retain its flavors.
When making Bolognese sauce, it is generally recommended to simmer it with the lid off. This allows the sauce to reduce and thicken, intensifying the flavors.
It is better to simmer with the lid off. The purpose of simmering the sauce is to reduce it, or to boil some of the water out of it so that the flavors in the remaining sauce are more concentrated. If you leave the lid on, then droplets of water will form on the underside of the lid and will fall back into your sauce, defeating the purpose of simmering for a long time.
This may depend on the recipe. Simmering with a lid on uses less heat than with the lid off. Leaving the lid on means that evaporation will be lower - but sometimes the purpose of simmering is to reduce the amount of fluid.... so this may be necessary.
Simmering can be done with or without a lid.
The ideal simmer temperature for cooking a stew is around 180-200F (82-93C).
Heat the pot back up, everything will expand again and the lid will come off.
To simmer is to bring something to a boil, reduce the heat and allow to boil slowly.
A simmer is when liquid is heated to a temperature just below boiling, around 185-205F (85-96C). The ideal temperature for achieving a simmer while cooking is around 200F (93C).
The ideal water simmer temperature for cooking pasta is around 212F (100C), which is the boiling point of water.
The temperature required to maintain a simmer while cooking is typically around 185 to 205 degrees Fahrenheit.
Uncovered in cooking means cooking with no lid or covering.
The use of a lid when cooking reduces the amount of heat escaping from the cooking area. Reducing the amount of time required for cooking and thereby reducing the amount of energy needed. This in turn reduces the amount of energy-required to pay the electricity bill. A good way to prove this point is to boil a pot of water with a lid and one without a lid and time how long they take to boil. Then see how much difference there is in the setting needed to make each simmer.