What exactly is an LDS mission?

If you're curious about what LDS missionary work is all about, this page provides a very good rundown.

In essence, she will be sharing the doctrines found in the Church of Jesus Christ of Latter-day Saints. Her purpose as a missionary is to "invite others to come unto Christ by helping them receive the restored gospel through faith in Jesus Christ and His Atonement, repentance, baptism, receiving the gift of the Holy Ghost, and enduring to the end." If you are interested in learning more, please visit www.mormon.org.