In control theory, when the output of the system is used as an input to the system to make it robust to variations and disturbances in the enviroment, components that are part of the loop, etc. The name comes from the fact that the output is "fed back" as an input, creating a loop of sorts.

It's mostly used to describe electronic circuits, but can also be used to describe biological systems. For example, there is a feedback loop to set the volume of your voice. The input is the desired volume level for your voice. The disturbance is the background noise in the environment you're in. The output is your voice. The feedback comes from your ears hearing your voice.

Deaf people have great difficulty controlling the volume of their voices without this feedback loop. It also shows up if you deliberately impair the feedback, for example, by drinking beer, or by using headphones.

Another example is a simple amplifier circuit found in a radio. Most amplifier circuits use transistors. There is a slight problem, though. An artifact of the manufacturing process is that the voltage gain on a transistor varies widely; for some models, from, say 50 to 500. Now, you can't test each transistor individually, and obviously that much variation is not going to lead to a stable design. But, by using a feedback loop, you can actually build an amplifier that is pretty much independent of the gain of the transistor, which means you guys can have nice cheap radios.