The Lamb broke the first of the seven seals. The ascended Lord Jesus, in full authority, triggers the series of awful events which will befall the earth and its population. It seems, although the events of chapter 5 followed immediately after the Ascension in Heaven, chapter 6 describes much later events on earth. From elsewhere in scripture (Matt 24.22; Dan 9.26-27), this entire period will be short; perhaps only seven years.
v1-8: The four horses, compare Mark 13.7-8, bringing warfare and famine on the earth, the beginning of sorrows. Each one is subject to the Lord, released by the Lord opening the seals (5.5). Their 'mission' was restricted, "do not damage the oil and the wine;" and the fourth horse had power over one-fourth of the earth. Yet, the sorrows delivered are significant.
Although such disasters happen throughout history. John is writing about the growing severity of these events towards the end of history. He also shows God's preservation of His people through all circumstances. These are real events in the future.
(I follow the 'futurist' view here; these chapters (6-20) largely describe future events. I do not think the seven seals and trumpets and bowls describe the same events: there are substantial differences. They seem to become more severe and less reversible, much like the ten plagues of Egypt.
v9-11: "Their fellow-servants and brothers," a reference to believers remaining on the earth; some will be killed before the final end, Matt 24.9-10.
There is justice from God, for their prayers will be answered. No accusation of unfairness can be made against Him, for He will "judge and avenge."
For the first readers, these are words of reassurance. Present suffering, even martyrdom, is not the final destiny. There is the God of justice, who will avenge.
v12-14: The sixth seal brought an earthquake. One of a number repeated in the book; 11.13, 19; 16.18.
v15-17: The great men of the earth are helpless before the mighty power of God, and the wrath of the Lamb, which is a fearful thing. The sixth seal brings about the "great day of His wrath," identified elsewhere as the Day of the Lord (Joel 2.31; Zeph 1.14-18).