(Note: It is suggested to read first the section on "Amplitude Modulation")
The first two results of YouTube for "frequency modulation":
The segment 06:56 – 10:24 is suggested. Comments below based on this.
In frequency modulation, the frequency of the carrier is modulated rather than the amplitude.
The carrier wave has constant frequency and amplitude.
The frequency of the carrier is called the rest frequency.
Let us consider that an audio signal is applied.
Below on the right of the red vertical line, in the absence of an audio signal (flat line) the carrier is unaffected. Notice what happens when the audio signal starts, and in particular when the negative half cycle begins.
The negative half cycle of the audio signal causes the carrier to drop below its rest frequency as evidenced by the wider space wave.
Notice what happens for the positive half cycle in the image below.
The positive half cycle of the audio signal causes the carrier to rise above its rest frequency, as evidenced by the narrow space wave.
In conclusion, the variation of the amplitude in the audio signal is accurately reflected by a frequency variation or modulation in the rest frequency of the carrier, as evidenced by the narrower or wider space wave patterns.