Binaural beats are imaginary sounds that the brain creates when two sounds at slightly different frequencies are heard in different ears. For example, if you hear a tone at 100Hz and another at 110Hz, the brain will recreate these tones in the brain stem at 105Hz, with a rising and falling amplitude at 10Hz.
This composite –the difference between the two original stimuli—will sound like a 10Hz beat. But the beat doesn’t exist anywhere except inside your head.