It is best if you read the page on the PHASE first.
Feedback is when some of the output signal from a circuit is fed back to the input and combined with the input signal.
If input and output signals are in phase then the feedback is POSITIVE.
If the two signals are out of phase then it is NEGATIVE FEEDBACK.
Positive feedback in an amplifier increases the gain and reduces the bandwidth of the amplifier.
If there is sufficient positive feedback then the amplifier will oscillate.
If a microphone is too near to a loudspeaker then you will get positive feedback causing "howl round".
Negative feedback reduces gain and increases bandwidth.