The Last Judgment is that time where all humanity comes before God to be judged. Those who pass will be with God in heaven for all eternity, while those who do not will be sent away to Hell. In Christian theology, only a personal knowledge of Jesus as Lord and Savior allows for the individual to pass into God's presence.
The timing of the two main judgement days, according to Premillennialism, occur right after the First Resurrection and right before the Millennium, and the other judgement occurs right after the Second Resurrection and after the ending of the Millennium, which happens before the Eternal State. The last judgement is called the Great White Throne Judgement. (See Revelation chp. 20)
Islam also has a form of The Last Judgment.