The Lion and the Lamb both refer to Jesus Christ. He is both the conquering Lion of the tribe of Judah and the Lamb who was slain.
An angel proclaims with a loud voice, “Who is worthy to open the scroll and break its seals?” John begins to despair when no one comes forth to answer the angel’s challenge. One of the 24 elders encourages John to “weep no more,” and points out that the Lion of the tribe of Judah has come to take and open the scroll.
Jesus is worthy because He has conquered sin and death at the cross. The cross was the ultimate victory of God over the forces of sin and evil.
So when Jesus is referred to as the Lion and the Lamb, we are to see Him as not only the conquering King who will slay the enemies of God at His return, but also as the sacrificial Lamb who took away the reproach of sin from His people so they may share in His grace.