Bandha means “lock” or “seal” and they are essential components of the ujjayi breathing technique (especially in Hatha yoga).
There are three classic bandhas located in the body: Mula, Uddiyana and Jalandhara
Mula bandha is the anal lock. Uddiyana bandha is the lower abdominal lock. Both bandhas “seal in energy, give lightness, strength and health to the body, and help to build a strong internal fire. Mula bandha operates at the root of the body to seal in prana internally for uddiyana bandha to direct the prana upwards through the nadis.
Jalandhara bandha is the throat lock, which occurs spontaneously in a subtle form in many asanas due to the dristi. This lock prevents pranic energy escaping and stops any build-up of pressure in the head when holding the breath.
According to yogic texts, the purpose of utilizing bandhas is to “unlock pranic energy and direct it into the 72,000 nadi [energy channels] of the subtle body.”
As always, for more information on the use of bandhas talk to your yoga teacher!