Quick example: Brainwave readings (with an EEG machine) have shown that brainwaves of 2Hz occur when an individual is asleep. Scientists can create special audio recordings, which contain an encoded 2Hz frequency. While listening to such audio, the brain naturally follows (entrains) toward that 2Hz frequency, lowering its brainwave patterns. That's "brainwave entrainment." The net result is that when listening to such an audio recording, you (the listener) will naturally fall asleep.
Neural oscillations are rhythmic or repetitive electrochemical activity in the brain and central nervous system. Such oscillations can be characterized by their frequency, amplitude and phase. Neural tissue can generate oscillatory activity driven by mechanisms within individual neurons, as well as by interactions between them. They may also adjust frequency to synchronize with the periodic vibration of external acoustic or visual stimuli.