Feedback
Think about a sound system with input from a microphone and output to a loudspeaker. If the sound coming out of the loudspeaker is heard by the microphone, a loop is created, from speaker, to the microphone, through the cord, and back into the speaker.  The loop is made up of sound and the electrical energy that the microphone and loudspeaker use to create that sound.  If nothing stops the sound and electricity in the loop, it will travel faster and faster, which is why feedback starts out as a low hum and turns into a high squeal very quickly