The four Yugas also known as four ages. According to Hindu scriptures and mythology, the current universe is destined to pass through four great periods, each of which is a complete cycle of cosmic creation and destruction. Hindu mythology deals with numbers large enough to be nearly impossible to imagine. Each yuga is progressively shorter than the preceding one, corresponding to a decline in the moral and physical state of humanity, and because the process of creation is cyclical and never-ending, it "begins to end and ends to begin."
A Kalpa, or eon, is said to be comprised of a thousand cycles of four yugas—each of a different quality. By one estimate, a single yuga cycle is said to be 4.32 million years, and a Kalpa is said to consist of 4.32 billion years
The four great periods in Hinduism are Satya Yuga, Treta Yuga, Dwapar Yuga, and Kali Yuga. Satya Yug or the Age of Truth is said to last for 4,000 divine years, Treta Yuga for 3,000, Dwapara Yug for 2,000 and Kali Yuga will last for 1,000 divine years—a divine year equalling 432,000 earthly years.
Hindu tradition holds that three of these great ages of this current universe have already passed away, and we are now living in the fourth one—the Kali Yuga. Reading about Kali Yuga and seeing what is going on in the world right now personally for me it explains a lot! …..but one cannot some it up in a simple sentence like that, as with everything else in Hinduism there are deeper and more symbolic meanings.
Another theory interprets these periods of time to represent the degree of loss of righteousness in the world. This theory suggests that during Satya Yuga, only truth prevailed (Sanskrit Satya = truth). During the Treta Yuga, the universe lost one-fourth of the truth, Dwapar lost one-half of the truth, and now the Kali Yuga is left with only one-fourth of the truth. Evil and dishonesty have therefore gradually replaced truth in the last three ages.
We are said to currently be living in the Kali Yuga—in a world infested with impurities and vices. The numbers of people possessing noble virtues are diminishing day by day. Floods and famine, war and crime, deceit, and duplicity characterise this age. Look at the world right now and this global pandemic.
Kali Yuga has two phases: In the first phase, humans—having lost the knowledge of the two higher selves—possessed knowledge of the "breath body" apart from the physical self. Now during the second phase, however, even this knowledge has deserted humanity, leaving us only with the awareness of the gross physical body. This may explain why humankind is now more preoccupied with the physical self than any other aspect of existence.
Due to our preoccupation with our physical bodies and our lower selves, and because of our emphasis on the pursuit of gross materialism, this age has been termed the Age of Darkness—an age when we have lost touch with our inner selves, an age of profound ignorance.
Both the two great epics—the Ramayana and Mahabharata—have spoken about the Kali Yuga. In the Tulasi Ramayana, we find the Sage Kakbhushundi foretelling:
In the Kali Yuga, the hotbed of sin, men and women are all steeped in unrighteousness and act contrary to the Vedas. Every virtue had been engulfed by the sins of Kali Yuga; all good books had disappeared; impostors had promulgated a number of creeds, which they had invented out of their own wit. The people had all fallen prey to delusion and all pious acts had been swallowed by greed.
In the Mahabharata (Santi Parva), the hero Yudhishthir says: The ordinances of the Vedas disappear gradually in every successive age, the duties in the Kali age are entirely of another kind. It seems, therefore, that duties have been laid down for the respective age according to the powers of human beings in the respective ages.
The sage Vyasa, later on, clarifies: In the Kali Yuga, the duties of the respective order disappear and men become afflicted by inequity.
After writing my book the 10 Avatars of Vishnu, Vishnu incarnates himself again to save to world and rid it of evil coming in the form of Kalki. According to Hindu cosmology, it is predicted that at the end of the Kali Yuga, Lord Shiva will destroy the universe and the physical body will undergo a great transformation. After the dissolution, Lord Brahma will recreate the universe, and humankind will become the beings of truth once again.
(Ref: Learn Religions)