The fidelity is the ability of radio receiver to produce all frequency components at output which are present at input.

The fidelity basically depends on the frequency response of the AF amplifier.

The typical fidelity curve is shown below:

High fidelity is essential in order to reproduce good quality music faithfully i.e. without introducing any distortion.

For this it is essential to have a flat frequency response over a wide range of audio frequencies.

The fidelity curve for a receiver is basically the frequency response of the AF amplifier stage in the receiver.