Question

What is feedback?

Answer 1

Feedback is information about reactions to a product, a person's performance of a task, etc. which is used as a basis for improvement.

Answer 2

PsyDAG gave you a great answer. There is a second definition of feedback in electronics. If a microphone is held too close to the output of the system (the speaker) there is "feedback" in the form of an audible squeal. The squeal may vary in intensity and even in frequency.