Anima is one of the eight principal siddhis (abilities or magical powers) that are only attained through prolonged meditation and yogic practices. This particular ability is that of the yogi being able to reduce their size until they become tiny, to the point of being as small as an atom.
Through this siddhi, the yogi can also change the density in their body and is said to be able to pass through solid objects as a result of this ability.
Reaching the stage of gaining the anima siddhi is something that can only be achieved through years of dedication to yoga, meditation and spiritual growth. However, those advanced yogis who gain anima, or any of the other main siddhis, rarely reveal their powers to other people.