A lot of people who aren't members of The Church of Jesus Christ of Latter-day Saints have heard about Mormons' temple garments. Some people refer to them as 'magic underwear,' which is completely inaccurate and somewhat offensive.
The church actually just came out with official video entitled "Sacred Temple Clothing" to explain to people not of our faith a little more about these garments. Please watch this video if you've ever wondered about our unusual clothing.
They express it much better than I ever could. Please if you are curious, investigate these things from the source, not from people who have only 'heard about it' or who have left the church.