“Jesus Christ taught that baptism is essential to the salvation of all who have lived on earth (see John 3:5). Many people, however, have died without being baptized. Others were baptized without proper authority. Because God is merciful, He has prepared a way for all people to receive the blessings of baptism. By performing proxy baptisms in behalf of those who have died, Church members offer these blessings to deceased ancestors. Individuals can then choose to accept or reject what has been done in their behalf.”

https://www.churchofjesuschrist.org/study/manual/gospel-topi...