While it is a phrase that doesn’t actually occur in the Scripture, “the Great White Throne Judgment” has become a way to describe this time (which is described in Revelation 20:11-15) where the Being who is judging those who stand before Him is described as occupying a Great White Throne.
When Christ returns to earth to begin his 1,000-year reign, those "who are His" at His coming (those who know, obey and follow Him now), and those who have died in the faith will be changed into spirit and will reign with Christ for 1,000 years (1 Thessalonians 4:13-18, 1 Corinthians 15:50-52, Revelation 20:4-6). This is referred to as the first resurrection.
Revelation 20:12-13 describes the events of a second resurrection, or a general resurrection of the dead. At this time, all of those individuals who have ever lived but did not understand, or never had the opportunity to know Jesus Christ—or the teachings of God—will be resurrected to physical life and have a lifetime to learn.
They will get the opportunity they never had to learn the way of God.
As they live their lifetime, God is watching. His kings and priests are teaching and helping these individuals to understand. During this time, and even now, Jesus Christ keeps a “book of life” which is referenced in Revelation 21:27 and numerous other locations, which contain the names of those individuals who will receive eternal life. Those whose names are blotted out will receive eternal death (Revelation 3:5, Exodus 32:31-32).
This Book of Life is opened during the time of the Great White Throne Judgment, and those individuals will ultimately stand before God as their lives are weighed and measured in accordance with the standard of Scripture. Figuratively, God will sort the sheep from the goats (Matthew 25:31-34).
Those who listen and follow the Shepherd’s voice in their life will receive eternal life in the Kingdom of God, whereas those who do not will be permanently destroyed in the Lake of Fire. A merciful and loving end for those who are unwilling to be a part of the Kingdom of God.
The Great White Throne Judgment is represented by the final of God’s festivals in his Holy Day plan, the Eighth Day. You can read more about the Eighth Day and its significance to the plan of God and man’s ultimate destiny here.