Negative Feedback is when your body is trying to reach a set point which is a is like a good level. An example of that would when your body tries to reach homeostasis. It can also be to reverse the change.
An example of negative feedback would be maintaining blood sugar levels. For someone without diabetes, a fasting blood sugar awakening should be under 100 mg/dl. Before meal normal sugars are 70-99 mg/dl. "Postprandial" sugars taken two hours after meals should be less than 140 mg/dl. Those are normal numbers for someone without diabetes.
Another example of negative feedback is thermoregulation. Your body has maintain a stable body temperature of about 98.6 F. If you are cold, you will shiver and if you are hot your body will sweat. This is all to maintain a stable body temperature.
Another example would be osmoregulation. Osmoregulation is the process of maintaining salt and water balance (osmotic balance) across membranes within the body. The fluids inside and surrounding cells are composed of water, electrolytes, and nonelectrolytes.