Rev 10:3

3 He cried with a loud voice, as a lion roars. When he cried, the seven thunders uttered their voices.

as when a lion roars – Christ is the Lion of Judah, as described in Rev 5:5.

seven peals of thunder uttered their voices – the meaning of the speaking of the seven peals of thunder seems to be revealed in Rev 19:6; in the phrase “Hallelujah! For the Lord our God, the Almighty reigns.”