Haggai was a contemporary of Zechariah, and his prophetic mission was to encourage the reconstruction of the temple by the returning exiles. Haggai focused on the importance of the temple to the Jews, urging them to make it’s reconstruction a priority. He also criticized materialism and self-centeredness, advocating a timeless message of making God the people’s priority and focus.
Haggai is sometimes called a minor prophet. This doesn’t mean he wasn’t important, but merely that the book named after him is shorter. (Compare this to the major prophets, whose books are much longer.)