The Fifth Seal

When the Lord opens the fifth seal, John has a view of the altar of the heavenly temple. Underneath the altar he sees “souls 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, dost thou not judge and avenge our blood on them that dwell on the earth?” (6:9-10).

Who are these souls? And why are they “under the altar” of the temple?