Yin yoga is a passive, yielding type of yoga. It’s postures are held for a long time, typically five minutes or more which allows a deeper type of stretch. By holding the postures for a longer time you can work more into the connective tissues, ligaments, joints, bones, fascia etc.
Yin is a simple practice but not easy. It can also help with the flow of energy (chi) through the body and also with the nervous system.