What is the millennium - the thousand-year reign?
Christians disagree on this one more than almost any other detail in the book, and reasonable, careful readers land in different places - whether Christ returns before the thousand years (premillennialism), after them (postmillennialism), or whether the number describes the whole church age symbolically (amillennialism).
Revelation 20 is also one of the few places in the whole book with almost no parallel elsewhere in the New Testament, which is part of why it’s so debated. One reading worth understanding: the passage may be looking backward, not forward - a symbolic recap of the age between Christ’s first and second comings, in which those who died faithfully (including, specifically, the martyrs beheaded for their testimony) are already reigning with him now, in a hidden but very real way. Whichever view you hold or are exploring, this text is trying to say something hopeful: that faithfulness, even to death, is never wasted - it’s already a kind of reign.