When the result of some process is used as the input to that process there is a feedback loop. Depending on what the process is, the feedback can result in a stable or unstable end result. When a microphone is brought near it's amplified speaker, the amplifier makes the sound louder, which the microphone picks up and delivers to the amplifier which again boosts the signal and so forth. This is positive feedback. A bouncing ball or swinging door is an example of negative feedback where the ball bounces (or door swings) to LESS that the previous amount (negative amplification or damping). The iris in your eye controls the amount of light entering depending on the amount that your eye sees.
