The book of Malachi is a record of the messages of the prophet Malachi, or “my messenger”. As with most prophets, Malachi’s message focuses on current events for the people of Israel, but also has a secondary meaning, anticipating the life of Jesus.
The book of Malachi addresses a people who believed God had forgotten them and let them down. Although true worship was re-established in the rebuilt temple, the associated blessings predicted by Haggai and Zechariah were yet to take place. Malachi encouraged that the reverent worship of God continue, despite the peoples’ discouragement over not seeing God’s promised blessings.
Malachi 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.)