First off, understand you will never find the word ‘millenium’ in the Bible.
But, just because it’s not there, doesn’t mean it isn’t true. It’s kind of like the word ‘sovereignty’. We never find the word sovereignty in Scripture, but we find plenty of verses that tell us God is in fact sovereign. Meaning that God has the wisdom, the power, and the authority to do anything He chooses with His creation.
Or like the word ‘rapture’, we never find the word ‘rapture’ in our English Bible but we know from 1 Thessalonians 4:17 its there. We get the word ‘rapture’ from the Latin translation of 1 Thessalonians 4:17.
The same is true for the word ‘millennium’. We don’t see millennium in our English Bible, it comes from the Latin translation of Scripture. Millennium comes from two Latin words: ‘mille’ which means ‘a thousand’ and ‘annum’, which means years.
The Millennium refers to a literal period of a thousand years.
Again, even though we do not see the word ‘millenium’ in our English translations, we know this period of 1,000 years is factual. We see the phrase “a thousand years” or “the thousand years” used six times here throughout Revelation 20. So when you see the phrase “a thousand years” or “the thousand years”, think millennium.
The Millennium refers to a literal period of a thousand years when Jesus will set up HIs earthly kingdom.
If there is nothing else you understand about the Millennium, understand this much: it lasts for a period of 1,000 years and the sole purpose of the Millennium is for Jesus to set up his kingdom on earth, bring peace to the earth, and reset the world. Now with that said, could Jesus set up his kingdom in less time? Yes. Why does Jesus take 1,000 years to set up his earthly kingdom? I don’t know. No one can answer that. We just have to trust his timing. I’m sure he has his reasons which are for him to know and us to find out.
The Millennium is a prelude to Heaven.
Keep in mind the Millennium is not Heaven. The Millennium leads into Heaven. It prepares the way for Heaven. What’s going on during the Millennium is a snapshot of what the New Heaven and New Earth is going to be like.