Simmering liquid is gently heated to the point when small bubbles form, rise, and then barely shimmer across the surface. For water, that happens at about 185°F (85°C), but it's the action of the liquid, not the temperature, that's important.
Simmering is a very common culinary technique that is often confused by the novice cook with boiling, which is unfortunate, for boiling can ruin foods that sing when simmered. Simmering in liquid gently cooks fragile foods and tenderizes tough ones; it is much more subtle than boiling, which can cause soft foods to turn to battered mush and chewy foods to toughen even further. Simmering will take proportionately more of the preparation time for most dishes: while recipes often call for liquids to be brought to the boil, things usually subside very quickly into a simmer. Even boiled eggs are really simmered.
Joy of Cooking informs me that the French say that simmering makes a pot smile. This is probably because French cooking is built upon a foundation of gorgeous stocks, soups, and sauces which are simmered to extract maximum flavour from top quality ingredients. The French pot knows that it is involved in a delicious endeavour, and beams approvingly. So should you as you simmer. A great technique to learn and use.