The samsara-cakra, the wheel of material existence.

“As soon as a living entity is victimized by material desires to lord it over material nature, he is subjected to the control of material nature, which is supervised by the Supreme Soul. The result is that one again and again makes plans and is baffled, but as foolish as he is he cannot see the cause of his bafflement. This cause is distinctly stated in Bhagavad-gita: because one has not surrendered to the Supreme Personality of Godhead, he must work under the control of material nature and its stringent laws (daivi hy esa gunamayi mama maya duratyaya). The only means of becoming free from this entanglement is to surrender to the Supreme Lord. In the human form of life, the living entity must accept this instruction from the Supreme Person, Krishna: sarva-dharman parityajya mam ekam saranam vraja. “”Do not plan to achieve happiness and drive away distress. You will never be successful. Simply surrender unto Me.”

Unfortunately, however, the living entity does not accept the Supreme Lord’s clearly stated instructions from Bhagavad-gita, and thus he becomes a perpetual captive of the laws of material nature. Yajnarthat karmano ‘nyatra loko ‘yam karma-bandhanah: if one does not act for the satisfaction of Krishna, who is known as Vishnu or Yajna, he must be entangled in the reactions of fruitive activities. These reactions are called papa and punya–sinful and pious. By pious activities one is elevated to the higher planetary systems, and by impious activities one is degraded to lower species of life, in which he is punished by the laws of nature. In the lower species of life there is an evolutionary process, and when the term of the living entity’s imprisonment or punishment in the lower species is finished, he is again offered a human form and given a chance to decide for himself which way he should plan. If he again misses the opportunity, he is again put into the cycle of birth and death, going sometimes higher and sometimes lower, turning on the samsara-cakra, the wheel of material existence. As a wheel sometimes goes up and sometimes comes down, the stringent laws of material nature make the living entity in material existence sometimes happy and sometimes distressed.”

A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Seventh Canto, Chapter 13 – Text 30