Shat Kriyas are yogic purification practices for cleansing the system. In Hatha Yoga, this is given paramount importance, more importance than Yama and Niyama. They argue that before the body should even begin to practice yoga, the body should be cleansed and purified first. e.g. - before your race your car on a track, you have to ensure the track is clean, obstructed and can take the rigors of a race car. More info here: 6 Shat Kriya