Yin Yoga is a slow-paced approach practice where poses are held for a longer period of time in a relaxed manner, up to 5 minutes or more.
There are around 20 classical poses (+ variations) in the Yin yoga repertoire which might appear easy at first glance, the challenge remains in keeping stationary while cultivating stillness. Yin Yoga evolves into 3 main different levels of understanding, relating to the 3 main aspects of human existence :

On the physical level Yin Yoga targets bones, joints and connective tissues. There is no aesthetic ideal to reach which makes the Yin Yoga approach very liberating, instead there is an emphasis on a functional approach.

On the energetic level the practice reconnects with its roots of Taoist yoga, and uses the ancient map and modern theory of the meridians, the long holds in Yin restore harmony in the subtle body by targeting the channels that run through the connective tissues.

On the emotional/mental level the Yin approach prepares the practitioner for meditation as one of the central intention of the yin practice is the cultivation of inner stillness and a keen sense of introspection, like Paul Grilley describes it : Yin Yoga is a Quiet Practice.