Mormons believe that when the Old-Testament prophet Malachi spoke of “turning the hearts of the children to their fathers” he was referring to the importance of performing vicarious ordinances in their behalf.
The apostle Peter taught that between Christ’s death and resurrection He visited and preached to the spirits in prison. “Prison” refers to the spirit-world experience of those who did not follow Christ in this life. (Note that the typical connotation of “prison” does not apply here. Many honest, sincere, wonderful people will find themselves in spirit “prison” simply because they never had the opportunity to learn of Christ’s true gospel or because they were sincerely dedicated to other belief systems.) Through the missionary work Christ initiated, as described in the verses below, many in the spirit world have chosen to accept Christ and now await baptism and other essential ordinances. Mormons perform these ordinances vicariously for their ancestors in LDS temples.