The garment worn by Latter-day Saints that is given them in the temple is a symbol of covenants, purity and faithfulness to Jesus Christ. It is a reminder that, as Adam and Eve were clothed by the Lord ["unto Adam also and to his wife did the Lord God make coats of skins, and clothed them" (Gen. 3:21)] so we too are clothed as a reminder of our promises to the Lord. It should not be surprising that clothing which represents promises to the Lord should be regarded as holy, especially to those who understand the role of the temple in ancient Israel.
The Lord commanded Moses to instruct Aaron and others in the making and wearing of clothing that was to be regarded as holy. Speaking of Aaron the Lord told Moses, "Bring his sons and put shirts on them; put sashes around their waists and tie caps on their heads" (Exodus 29:8-9, Good News Bible). These garments were designated by God and considered sacred (Exodus 28, 39:41).
Elder Boyd K. Packer, a member of the Council of the Twelve Apostles, had some interesting comments about the undergarment received in the Temple. In one address to the non-member faculty and staff of the Navy Chaplain's Training School, he reminded them that as chaplains, they too wear articles of clothing that set them apart from everyone else. He added that their religious clothing meant a great deal to them and that likewise we draw something of the same benefits from our special clothing "as you would draw from your clerical vestments. The difference is that we wear ours under our clothing instead of outside. For we are employed in various occupations in addition to our service in the Church" (The Holy Temple, p. 76).
The wearing of the garment, although seen by some non-members as a peculiar
practice, is for the devoted Latter-day Saints an activity which symbolizes
a life devoted to Christ-centered activity. "He that overcometh, the same
shall be clothed in white raiment" (Revelation 3:4-5). Certainly it is
a practice for which there are several significant Biblical precedents.