Bandhas are Yogic techniques for locking the prana(life force) in our body. These are are done with certain breathing exercise or asanas for creating a more intensive meditative experience and awakening the Kundalini. While these Tantric techniques are thousands of years old but were formalized in Hatha Yog Pradeepika.
The Bandhas help advance practitioners to pierce the knots or granthis which prevent the Kundalini energy from rising through various chakras. These three knot are Brahma, Vishnu and Rudra are respectively pierced by Mool, Udyana, and Jalandhar bandhs. The first granthi deals with lower two chakras(Mooldhara and Swadishtana) and our attachment to desire. The second deals with the middle two chakras(Manipura and Anhata) and our attachment to action. The third deals with the upper two chakras (Vishudha and Ajna) and our attachment to thoughts. Once these knot are pierced Kundalini rises and prana flows unimpeded from root to crown chakra.