Malachi was a prophet (holy man). The Book of Malachi is the last book in the Bible's Old Testament.
Malachi said that God accused the people. They were doing many wrong things. They did not give the best gifts to God (Malachi 1:6-14). The priests did not teach well (Malachi 2:1-9). The men divorced their wives without a proper reason (Malachi 2:13-16).
But God himself would come to his temple (the house of God) (Malachi 3:1-5). But before this, God would send his special servant. This servant would teach the correct way to serve God. Then, the people would be ready for God's arrival. This was good news for the people who respected God. But this was terrible news for the people who refused to obey God (Malachi 4).
Malachi's book was written after all the other books in the Old Testament. 400 years later, Jesus came. But before Jesus began his work, a special servant of God taught near the river Jordan. This servant of God was called John the Baptist. John taught the people that they must confess their evil deeds to God. He taught that they must prepare themselves for the arrival of the Christ. (The 'Christ' means God's king. Many Old Testament prophets taught that the Christ would save us from the punishment for our evil deeds.)