Anything millenarian relates to the Christian idea that the Second Coming of Jesus Christ will bring 1000 years of peace and joy.
A millennium is a thousand years, so anything millenarian relates to one thousand years — but not any thousand years. This is a specifically Christian concept relating to the Second Coming of Jesus Christ. In some sects of the Christian religion, it's believed that the return of Jesus will lead an amazing new era that will last a thousand years. These are millenarian ideas, and they're part of millenarian theology. A novel based on these ideas could be called millenarian fiction.