Laghima Siddhi is one of the eight yogic powers or ashta siddhis. It is mentioned in Vyasa Bhasya on Yogasutra of Patanjali (III:45). The definition of Laghima given in Yoga texts is – becoming very light in weight like cotton fiber. On who has attained the Siddhi will be capable of free movement in space. Gravitation is not a problem for the person who has mastered Laghima.
To attain the power to fly the Yogi has to have the control of the five elements – earth, water, fire, air and space.
The yogi attains the power by the practice of Samyama (i.e., dharana, dhyana, and Samadhi, all the three together) on the qualities or special characteristics of five elements, which are grossness, nature, subtle, cause and effect relation and purposiveness.
Source – Encyclopedia of Hinduism Volume VI page 226 published by India Heritage Foundation