“Premillennialism comes from a term that means, literally, “before the thousand years”. Thus, it is essentially a way of interpreting Revelation 20, which six times mentions a period of a thousand years, during which Satan is bound and believers reign with Christ. Premillennialists believe that Christ will return and establish his Kingdom on earth, and that he will be visibly present for a thousand years, reigning over all the earth in an age of peace and prosperity. Then, after this thousand-year visible reign of Christ on earth, he will quell a final rebellion, enter into his last judgment, and finally usher in the eternal state, with its new heavens and new earth.
Premillennialists usually take a linear view of Revelation chapters 4 through 22, seeing all the visions and events spoken of in those chapters, not as different ways of describing the same period of time, but as one chronologically unfolding prophecy. Hence, since Christ’s thousand-year reign is spoken of after all the events leading up to chapter 20, it must take place after all these prophesied events. Thus, it is still future. Christ’s reigning for a thousand years is also understood to be speaking of his visible presence on the earth, and so he will return to the earth, sometime in the future, but before the thousand years.
Have more questions? Monergism is a great resource, check it out.