Mudras and bandhas are subtle techniques usually introduced once students have some experience of yoga practices.
A mudra is a specific gesture, shape or attitude. The subtle physical movements that make up the gesture can alter the flow of energy in the body in order to bring about changes in mood and perception, deepening awareness and concentration. Most commonly mudras are made by adopting certain positions of the fingers and hands - these are called hasta mudras (hasta = hand in Sanskrit). They are used in meditation practice. You're quite likely to meet chin mudra - index finger and thumb touching, or index finger sitting into the fold of the thumb, quite early on in a Satyananda class as we sit for a brief meditation at the beginning and chin mudra will be explained to you. For more experienced students a mudra can involve the whole body in a particular asana, with breath awareness, bandha and visualisation techniques.
Bandhas aim to lock prana in particular areas of the body and redirect it for the purpose of spiritual awakening. The Sanskrit word bandha means to hold, tighten or lock. Bandhas can be used in asana work but are more often associated with pranayama practice because their action is to enhance energy or prana with a view to progression in pranayama and meditation practice.