The bandhas are hydraulic locks that, once applied, allow you to access more of your infinite supply of energy (AKA "kundalini").
Kundalini energy is the potential, creative energy stored at the base of the spine. Everyone has the potential to awaken this energy & all yogas are intended to do so. Bhandas allow the kundalini energy to awaken & flow freely. Exactly like a hydraulic lock, one applies pressure so that the energy can pump upwards.
Each lock (root lock AKA mulbandh, uddiyana at the diaphragm, & jalandhar at the neck) is incredibly potent. Yogi Bhajan said it takes 18 months to perfect the most complex lock, jalandhar bandh.