Oh, I totally know the answer to this one!  In 1 Peter 3:19, it says that he went to visit and teach the spirits that were in spirit "prison."  There are people that die here on Earth without the knowledge of the restored gospel of Jesus Christ or who were not baptized, etc.  They too are children on God and since God is not a partial God and "does not esteem one flesh above another," He has prepared the way that all may be "partakers of Salvation."  In short, Christ went to the spirit world to visit the spirits who were in prison (1 Peter 4:6).\n\nIn trying to write my response, I realized that it could get kind of lengthy.  But here is a website with a quick summary to answer the question, complete with Biblical references: http://scriptures.lds.org/gss/slvtnfrt  \n\nAlso, for a complete explaination you can contact representatives or missionaries from the Church of Jesus Christ of Latter-day Saints (see www.mormon.org or call 1-800-645-4949) or ask one of your Mormon friends. :)