If some of the output of an amplifier is made to be out of phase, and mixed back with the amp’s input signal, it will partially cancel the input, reducing the gain of the amplifier; this is called negative feedback.
But, because it contains and therefore cancels any distortion introduced by the amplifier, negative feedback also has the effect of improving the linearity of the amplifier. Negative feedback can also lower output impedance, increasing damping factor, and can sometimes be made to flatten frequency response.
The key to negative feedback amplifiers is careful design. Too much phase shift and the amp will be unstable, and too much feedback will cause Transient Intermodulation Distortion.