What does Revelation 6:10 mean?

The prior verse mentioned souls who had been slain for their witness of the Word of God. According to this verse, the souls under the altar cry out for vengeance. They ask God how long it will be before He judges and takes vengeance on those who shed the martyrs' blood.

Their prayer may be classified as an imprecatory prayer, a prayer that calls for vengeance. This kind of prayer is recorded in Scriptures such as 2 Chronicles 24:22; Psalms 54:5; 68:22–23; 79:6; and 143:12. This connection, and chapter 7's description of believers "sealed" by God, suggests these martyrs are Jewish believers converted during the time of the tribulation, not present-era Christians. Believers of the present, the "church age," are told to respond to maltreatment with kindness. When Stephen, a church-age believer, was being martyred, he exemplified the manner in which Christians ought to respond to maltreatment. He cried out loudly, "Lord, do not hold this sin against them." (Acts 7:60).

The souls under the altar recognize God's absolute right to have allowed their martyrdom. They address him as "Sovereign." They also attest to His holy character. Being holy, God cannot commit any wrongdoing. Therefore, God did nothing wrong in allowing their martyrdom. Further, they address God as "true," literally, "perfect." He did not, nor could He, make any mistake in allowing these tribulation believers to be martyred.