Most temples have a golden “angel Moroni” statue on top.
Mormons believe that Moroni was an ancient prophet on the American continent. Moroni was sent as a messenger to Joseph Smith, and directed him to the golden plates from which the Book of Mormon was translated.
Mormons see Moroni as the fulfillment of one of the angels mentioned in the book of Revelation in the Bible:
I saw another angel fly in the midst of heaven, having the everlasting gospel to preach unto them that dwell on the earth, and to every nation, and kindred, and tongue, and people, saying with a loud voice, Fear God, and give glory to him; for the hour of his judgment is come: and worship him that made heaven, and earth, and the sea, and the fountains of waters (Revelation 14:6-7)
The angel statue thus represents the preaching of the gospel of Jesus Christ to all nations, calling all to worship and give glory to God, in preparation for the second coming of the Lord.
Not all temples include this feature (e.g., the Cardston, Alberta temple does not). The inclusion of Moroni is an architectural and stylistic decision; it is not a feature of worship or a requirement on all temples.