Revelation 10:3 says: And [he] cried with a loud voice, as when a lion roareth: and when he had cried, seven thunders uttered their voices.

This is the cry of the Lion of the tribe of Judah (see Hebrews 7:14). Immediately prior to executing the judgments listed in the book, He cries loudly (or roars as a lion) to warn of impending danger. Other portions of Scripture also speak of His roaring when He comes as the Judge of the universe. Hosea 11:10 states: They shall walk after the Lord: he shall roar like a lion: when he shall roar, then the children shall tremble from the west. Joel 3:16 adds: The Lord also shall roar out of Zion, and utter his voice from Jerusalem; and the heavens and the earth shall shake: but the Lord will be the hope of his people, and the strength of the children of Israel. Again, Amos 1:2: The Lord will roar from Zion, and utter his voice from Jerusalem.

When the Lord roars, seven thunders utter their voices. Though thunder is usually associated with judgment, no attempt will be made to explain the meaning, since God forbids it in verse 4.

Verse 4: And when the seven thunders had uttered their voices, I was about to write: and I heard a voice from heaven saying unto me, Seal up those things which the seven thunders uttered, and write them not.

Well, someday we will know! For the present, however, God commands that this one portion of Scripture be kept secret.