The human brain contains about 100 billion nerve cells called neurons. Neurons are electrically excitable cells that communicate with other cells through connections called synapses. When these neurons are communicating, they form synchronized electrical pulses which produce brainwaves.
Brainwaves are patterns of electrical activity that occur in the brain and different brainwaves have different frequencies (slow, fast, etc). The frequency of a brainwave helps us tell one brainwave pattern apart from another. Brainwave frequencies are measured in Hertz (Hz) which looks at how many cycles of a brainwave pattern occur each second. For example, a 1 Hz wave means there is only one wave occurring each one second, while a 20 Hz wave means there are 20 waves occurring in one second. Therefore, delta waves (0.5-3.5 Hz) are much slower than beta waves (13-38 Hz).