Answer
Broadly speaking, Micah's prophecies primarily concerned the state of Israel and Judah around the time of the fall of Samaria (722 B.C.). The mention of Hezekiah places the date of the book after this event, and the listing of the three kings of Judah (Mic. 1:1) indicates that the Judahites were his main audience.
Thematically, the book emphasizes oracles of judgment and blessing in order to motivate God's people in Judah to obey God in order to avoid receiving the curses of the judgment oracles, and to gain the rewards of the oracles of blessing. It also emphasizes the restoration of the kingdom under the leadership of a messianic figure. This message is relevant for Christians because it typifies the judgment that God metes out to sinners, and the salvation that Christ brings. In Christ's first coming, he inaugurated his kingdom -- the very kingdom of the restoration that Micah offered -- which now continues to grow and which will reach its consummation at Christ's return. Besides providing this broad view of redemptive history, the book includes many practical applications in the form of instructions for godly living, and offers God's kingdom blessings to those who will obey. One good way to think about application of the prophets in general is to note those aspects of the prophecies that Christ has already fulfilled (in which we may find security), those aspects which characterize our current lives (by which we may be sanctified, encouraged, and exhorted) , and those aspects which relate to the future kingdom in its fullness (in which we may take great hope).
9th-grade summary: If God's people disobey God, God may punish them and not bless them. If they obey him and follow his chosen leader, however, God will grant them incredible blessings. In our modern churches, we apply these ideas mainly by relating them to Jesus. If people reject Jesus and disobey God, God will judge them eternally in hell. But if they are faithful to Jesus, they will receive all of God's blessings in heaven, and in the new heavens and new earth.
Answer by Ra McLaughlin
Ra McLaughlin is Vice President of Finance and Administration at Third Millennium Ministries.