Three Angels' Messages
First Angel
"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 KJV)
The first angel's message is a prophecy that states that the gospel will be preached to every nation on earth.
Second Angel
"There followed another angel, saying, 'Babylon is fallen, is fallen, that great city, because she made all nations drink of the wine of the wrath of her fornication.'" (Revelation 14:8 KJV)
The second angel's message is a call to repentance. Broadly taken, it is a call to everyone everywhere to leave their sins ("Babylon") behind them and give them up.
Third Angel
"The third angel followed them, saying with a loud voice, 'If any man worship the beast and his image, and receive his mark in his forehead, or in his hand, the same shall drink of the wrath of God, which is poured out without mixture into the cup of his indignation; and he shall be tormented with fire and brimstone in the presence of the holy angels, and in the presence of the Lamb: and the smoke of their torment will ascend for ever and ever: and they have no rest day nor night, who worship the beast and his image, and whosoever receives the mark of his name. Here is the patience of the saints: here are they that keep the commandments of God, and the faith of Jesus.'" (Revelation 14:9-12 KJV)
The third angel's message is a warning to the people of the earth, not to worship the beast or his image. (The "beast" and his "image" were introduced in chapter 13 of Revelation.) Those previous verses are the source of the famous mark of the beast .