The main theme of the Book of Revelation, is the Second Coming of Jesus Christ. The word “cloud” in this verse is referring to great numbers of saints. When Jesus sets foot on the Mount of Olives, billions of people will see Him. At that time, even the Jews will know Jesus is their Messiah. The “wailing” spoken of here, is because of the Judgment Jesus will bring upon the world for all its sin. It is up to us to make sure we are ready for the Lord when He comes in the Rapture.
(Read Revelation 21:6)