When the Lamb opened the fifth seal, the souls of dead believers appealed to God to judge those who killed them, but God told them to rest a bit longer.

Revelation 6:9-11

9 When the one who is like a lamb opened the fifth seal, I saw under (OR, at the base of) the altar in heaven the souls of God’s servants who had been killed {whom people had killed} because of their believing God’s message, and because they told others the message about Jesus. 10 They spoke loudly to God saying, “Sovereign/Almighty Lord, you are holy and faithful. How long will it be before you judge and punish the people on earth who murdered us(exc)?” 11 God gave to each of them a long white robe, and he told them to rest a little longer until people killed all the believers who served the Lord with them. They were believers whom God wanted to die in just the same way that these others had been killed.