"When He opened the fifth seal, I saw under the altar the souls of those who had been slain for the word of God and for the testimony which they held. And they cried with a loud voice, saying, 'How long, O Lord, holy and true, until You judge and avenge our blood on those who dwell on the earth?'" (Revelation 6:9-10 Revelation 6:9-10  And when he had opened the fifth seal, I saw under the altar the souls of them that were slain for the word of God, and for the testimony which they held:
 And they cried with a loud voice, saying, How long, O Lord, holy and true, do you not judge and avenge our blood on them that dwell on the earth?
American King James Version×).
To understand this scripture, we must remember the context. John was witnessing a vision while he was "in the Spirit" (Revelation 4:2 Revelation 4:2And immediately I was in the spirit: and, behold, a throne was set in heaven, and one sat on the throne.
American King James Version×). Under inspiration he was seeing future events depicted in symbolism. The fifth seal is figurative of the Great Tribulation, a time of world turmoil preceding Christ's return.
In this vision, John sees under the altar the martyred believers who sacrificed their lives for their faith in God. These souls figuratively cry out, "Avenge our blood!" This can be compared to Abel's blood metaphorically crying out to God from the ground (Genesis 4:10 Genesis 4:10And he said, What have you done? the voice of your brother's blood cries to me from the ground.
American King James Version×). Though neither dead souls nor blood can actually speak, these phrases figuratively demonstrate that a God of justice will not forget the evil deeds of mankind perpetrated against His righteous followers.
This verse does not describe living souls that have gone to heaven. The Bible confirms that "no one has ascended to heaven but He who came down from heaven, that is, the Son of Man who is in heaven [Jesus Christ]" (John 3:13 John 3:13And no man has ascended up to heaven, but he that came down from heaven, even the Son of man which is in heaven.
American King James Version×). Even righteous King David, a man after God's own heart (Acts 13:22 Acts 13:22And when he had removed him, he raised up to them David to be their king; to whom also he gave their testimony, and said, I have found David the son of Jesse, a man after my own heart, which shall fulfill all my will.
American King James Version×), was described by Peter as being "dead and buried" (Acts 2:29 Acts 2:29Men and brothers, let me freely speak to you of the patriarch David, that he is both dead and buried, and his sepulcher is with us to this day.
American King James Version×), not alive in heaven or some other state or location (verse 34).
The description of souls "under the altar" is obviously symbolic. In 1 Thessalonians 4:16 1 Thessalonians 4:16For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first:
American King James Version×Paul makes it clear that deceased Christians remain in the grave until Christ returns. When we allow the Bible to interpret itself, Revelation 6:9 Revelation 6:9And when he had opened the fifth seal, I saw under the altar the souls of them that were slain for the word of God, and for the testimony which they held:
American King James Version×cannot mean that anyone is in heaven now. Rather, it is both a prophecy that martyrdom would occur and that it would continue until the second coming of Christ.
You can read more about how this section of Scripture fits into the overall framework of Bible prophecy in our booklet The Book of Revelation Unveiled under the section about "the fifth seal."