690Hz is a popular tune in Rife community and is believed to be a cure for many diseases like abscess, pain, allergy, appendicitis, bacterial and viral infections, parasites, lyme, stomach disorders, thyroid fever and is a general antiseptic too.
In this video 690Hz is modulated with 6 extremely low frequencies from 0.1Hz to 0.01Hz which comes under below delta range. So it is very soothing and relaxing. This brainwave tones can take brain to a deep sleep like level and healing and such activities will be at its maximum when brain moves to delta and below delta ranges.
Please note this is not a replacement for general medical treatment. If you have any serious health issues please consult a doctor first. These kind of alternative things can be used in addition to your primary treatment.