The seals are opened and represent the coming overwhelming judgment of God, and we see here the difficulty in interpreting the images in Revelation. In 6:9 we see souls under the altar crying out for justice. Why? Probably because that is where the blood of the sacrifices would pool when they were offered in the temple. And the apocalyptic imagery from 6:12-14 mirrors Old Testament imagery but may not be strictly literal. It could simply be illustrating the coming catastrophic judgment.
Four seals bring four horses, signifying conquest, war, famine, and death. Horses are used here because they are illustrative of tactics and warfare at the time. If Revelation were written in the 20th century tanks may have been used! The first horse brings to mind the Parthenian’s, who were known to retreat on horseback and use their bows to fire backwards and devastate the pursuing armies. The comments on famine (third seal), show that it could be worse. Armies would often destroy the crops in the fields to temporarily devastate a region, but crops that took longer to come to fruition were left alone. The overall summary of the devastation caused by the horsemen also reveals that God is at work restraining them: they only have authority to harm a fourth of the earth. Even the comments about the martyrs reveal that God is in control! They cry out for justice, but it won’t happen until the time is right: “Each of them was given a long white robe and they were told to rest for a little longer, until the full number was reached of both their fellow servants and their brothers who were going to be killed just as they had been.” The overall upheaval is so bad that even unbelievers recognize that judgment is coming, and long to hide from it.
There are two lessons that should jump out at us. The first is that there is a coming judgment against the world, and it will be devastating. No-one can escape. The second is the reminder that no matter what happens and no matter how horrible it seems, God is in control. He has a purpose and a plan.
For worship: https://www.youtube.com/watch?v=H1hcRN16y7E