The gathering of Zion
The LDS church believes in a literal gathering of the disciples of Christ before the second coming takes place. For those new to the term "second coming," it is when Christ will come to Earth to visit when the gospel is taught to all the Earth. When this happens all the unrighteous will be destroyed and a time of peace and prosperity will take place for the course of 1,000 years. This time of peace is referred to as the Millennium. Do not worry however because Dallin H. Oaks of the Quorum of the Twelve Apostles of the Church of Jesus Christ of Latter Day Saints gives the scripture D&C 38:30 to tell us that we need not fear if we are prepared. During the time in the Millennium family history work will be done to perform all the ordinances (such as baptism or the sealing of marriages) for those that have died or are still alive that did not have the chance and would have if they could. This is why Temples are on the Earth today. Sacred ordinances that we need to perform in order to return to our Heavenly Father is done by these sacred houses of God.