The two frequencies, sum and difference frequencies, generated during the heterodyning process or during the amplitude-modulating process. For example, if a 500 kHz carrier signal is amplitude modulated with a 1 kHz frequency, the beat frequencies are 499 kHz and 501 kHz.
A good demonstration of beat frequencies can be heard in the animation below. A pure sound of 330 Hz is combined with 331 Hz to give a rather slow beat frequency of 1 Hz or 1 fluctuation in amplitude per second. When the 330 Hz sound is combined with a 340 Hz sound, you can hear the more rapid fluctuation at 10 Hz.