Bandha is a Sanskrit word meaning "lock," "seal," or "bind" and this is a practice you use for yoga.

Moola bandha (root lock, like the pelvic floor), Uddiyana bandha (abdominal lock), and Jalandhara bandha (chin lock) are muscular contractions you hold that can help protect you from injury and over stretching. These 3 are used in Ashtanga yoga to control the energy built up inside the body. They also add strength and stability to poses and help to to build your internal fire. They also help you breathe properly. They are used in other forms of yoga as well.

For further reading, check out: Yoga Explained: Bandhas