Haggai was a prophet (holy man). He lived at the same time as Zechariah. Then, some people from Israel had returned from Babylon. They rebuilt their own houses in Jerusalem. Haggai explained why they were poor. He said that God was not blessing them.
Haggai told them that they should also rebuild God's temple (the house of God) - Haggai 1.
The new temple was small. But God told Haggai that it would be greater than the old temple (Haggai 2). And now, God would bless the people because they had rebuilt the temple.