John hears about a lion who has overcome and has conquered in 5:5. But when he turns, what he sees is not a great warlike lion at all, rather a lamb, the most docile of creatures, and specifically, a slain lamb, which is epitome of weakness. So Revelation has the same message as the rest of the NT; God overcame sin by the cross; His power was perfected in weakness. It’s not talking about a literal lion, it’s not talking about a literal lamb, but it’s using symbolic language to communicate a point.