Kalki is the tenth and final Maha Avatar (great incarnation) of Vishnu who will come to end the present age of darkness and destruction known as Kali Yuga. The name Kalki is often a metaphor for eternity or time.
The Appearance of Lord Kalki, the Avatar or incarnation of Godhead, is foretold in the Srimad Bhagavatam 12th Canto and described also in detail in the Brahmanda-purana. Therein it gives predictions as to where he will appear (the village name), who his parents will be, what his mission will be, and when he will appear.
It is described that he will appear at the conjunction of the two yugas, namely at the end of Kali-yuga and the beginning of Satya-yuga. The great cycle of the four yugas, namely Satya, Treta, Dvapara and Kali, rotates like the calendar months. The present age of Kali-yuga lasts 432,000 years, out of which we have passed only 5,000 years after the Battle of Kuruksetra and the end of the regime of King Parikshit. So there are 427,000 years balance yet to be finished, till the advent of Lord Kalki. Therefore at the end of this period, the incarnation of Kalki will take place, as foretold in the Srimad-Bhagavatam. The name of His father, Visnu Yasa, a learned brahmana, and the village Sambhala are also mentioned.
By the End of Kali Yuga, the population of the whole world will be so much reduced to animalism that there will be no more power to understand what is God, or what is spiritualism. And it is already there, the Kali-yuga. It will increase. People will have no power to understand this philosophy, God consciousness. So at that time, when there are no more religious principles, there is no other alternative than to kill them all.
At that time the Kalki avatara will come, and He'll take a sword on the horseback, just like a king send from heaven, He'll simply go on killing all these non-believers, Godless creatures and usher in another Satya-yuga, the golden age of righteousness and truth
The mission of Kalki Avatara is to re-establish the religious principles, which have disappeared by the end of Kali Yuga, the iron age of hypocrisy and quarrel.