Hold the microphone up to the speaker. It will shriek. The mic has picked up the speakers hissing and crackling, and fed it back, picked that sound up and fed it back. It does this again and again, at nearly the speed of light, until the sound is unbearably loud and high-pitched.
You could make a thermostat that raised its setting every time the temperature reached the prior setting. Eventually your house would be a sauna.
Both of these are examples of positive feedback.