A negative feedback mechanism is a system whereby the trigger for the production of a particular substance is the measured level of the substance itself. In other words, if the concentration of a substance is measured to be too low, this triggers the machinery that makes that particular substance. In the same vein, when the concentration of the substance is measured to be in the correct range, this triggers the production of that substance to stop.
The endocrine system is a bodily system that produces hormones, or small chemical molecules that regulate many different processes in the body including growth and metabolism. Negative feedback loops help to control the hormone levels in the body. When a particular hormone level is too low, this triggers the production of that hormone. When the hormone level is back to normal, the production of that hormone is stopped. This helps to body to maintain fairly constant levels of hormones at all times.