Siddhi means perfection of life. Generally they take it asta-siddhi of yoga practice -- anima, laghima, mahima, prapti, siddhi, isitva, vasitva, prakamya. So these are called siddhis, yoga-siddi. Yoga-siddhi means you can become smaller than the smallest. Our actually magnitude is very, very small. So by yoga-siddi, in spite of having this material body, one yogi can come to the smallest size, and anywhere you keep him packed, he will come out. That is called anima-siddhi. Similarly, there is mahima-siddhi, laghima-siddhi. He can become lighter than the swab of cotton. The yogis, they become so light. Still there are yogis in India. Of course, in our childhood we saw some yogi, he used to come to my father. So he said that he could go anywhere within very few seconds. And sometimes they go early in the morning to Jagannatha Puri, to Ramesvaram, to Haridwar, and take their bathing in different Ganges water and others. That is called laghima-siddhi. He became very light. He used to say that "We are sitting with our guru and just touching. We are sitting here, and after few seconds we see in a different place." This is called laghima-siddhi.
So there are many yoga-siddhis. People become very much puzzled by seeing these yoga-siddhi. But Krsna says, yatatam api siddhanam: (BG 7.3) "Amongst many such siddhas, who have got yoga-siddhi," yatatam api siddhanam kascid vetti mam tattvatah (BG 7.3), "somebody may understand Me." So one may achieve some yoga-siddhis; still it is not possible to understand Krsna. That is not possible. Krsna can be understood only such persons who has dedicated everything to Krsna.