In Satyayuga, Lord Vishnu incarnated and established truth, peace, mercy, forgiveness and friendship in the world. At that time everyone was knowledgeable about the scriptures and everyone was living life according to the Vedic tradition. At that time, the sages and sages had become egoistic and arrogant in terms of knowledge and due to that sin, Satyayuga came to an end. In Treta Yuga, Lord Shri Ram incarnated and people benefited from the blessings of Lord Shri Ram through Yagya etc. virtuous deeds and at the end of Treta Yuga, he destroyed the great sinners like Ravana and finally Khand Pralaya took place.
Again humans moved from Treta Yuga to Dwapar Yuga and the devotees of Golak Dham where they were born in Dwapar Yuga, they returned to Golak Baikunth along with the body parts of Lord Shri Krishna when Lord Shri Krishna sacrificed his body. At that time Kali Yuga had passed 1200 years and Kali spread everywhere due to the influence of his body. There is a verse in Bhagwat related to this:-
"Yada devarshayah sapta maghashu bicharantihin,
Tada prabrittastu kali dvadashardda – shataatkah.”
i.e. :-
When the seven sages were wandering in Magha Nakshatra, at that time ("Till the time of Shri Krishna's death") Kaliyuga had lasted 1200 years.
After this, Maharaja Parikshit dies and after that, the entire Kaliyuga started and Kali spread his influence throughout the universe. In this era people will become greedy, attachment, lust, anger, ego, prostitution and lazy and even though people have knowledge of Shastras, Puranas and Vedas, people will be anti-Shastras and anti-Vedas. Those who misunderstand religion, oppose the Vedas, commit sins like killing animals, consume intoxicants, oppose the gods, are Mlechchha in Kaliyuga.
Shri Jaidev ji has written in Geet Govind :-
"Mleccha-Nivah-Nidhane Kalayasi Karvalam dhoomketum ev kim api karalam Keshav Dhrit-Kalki-Sharir Jai Jagdish Hare।"
To destroy these evil sinners and Mlechchas, Lord Kalki will incarnate and will take a fierce form like a comet.
"Jai Jagannath"
