The Minor Prophets refers to the Biblical Books of Hosea, Joel, Amos, Obadiah, Jonah, Micah, Nahum, Habakkuk, Zephaniah, Haggai, Zechariah, and Malachi in the Old Testament. They cover the time period from about 800 to 400 B.C. and deal with historical time periods in Israel's history and, of course, prophecies or messages to the people either about their current time or the future.
The word Minor refers to the short length of these books, and does not refer to their importance. Taking the Bible as the word of God for humanity as many do, then every book of the Bible is there for a reason and is important.