Brainwave entrainment is basically the synchronization of a source of an external periodic stimulus and the dominant brainwaves of our brain.
The stimulus may be aural, as in the case of binaural beats, monaural beats, and isochronic tones, or else visual, as with a lights, a combination of the two with a mind machine, or even electromagnetic radiation.
By using brainwave entrainment deliberately we can achieve desired a brain-state such as arousal or relaxation. It depends upon the "frequency following" response, a naturally occurring phenomenon where the human brain has a tendency to change its dominant EEG frequency towards the frequency of the external stimulus.
Binaural beats deserve special mention because of the manner in which the desired frequencies are obtained. Brainwave entrainment may be achieved when audio signals are introduced to the brain causing a response directly related to the frequency of the signal introduced, called binaural beats.
For example, two tones close in frequency generate a beat frequency at the difference of the frequencies. For example, if a person receives a 300 Hz tone in the right ear and a 305 Hz tone in the left ear. The brain will generate a subsonic 5 Hz tone, roughly in the middle of the theta range. The resulting subsonic tone may affect the state of mind of the subject these frequencies, in this case provokins a deep relaxation.
For that reason it is said that brainwave entrainment through binaural beats can provide help in treating certain mental afflictions.