Then the angel took the censer, filled it with fire from the altar, and hurled it on the earth; and there came peals of thunder, rumblings, flashes of lightning and an earthquake.Revelation 8:5 NIV
The first 5 verses of chapter 8 seem to mark a turning point in John’s vision. It starts with a brief time of quiet anticipation in the normally praise filled heaven. Following this is the introduction of 7 angels with trumpets who will occupy the next portion of the vision. An 8th angel then comes before the altar first seen in Revelation 6:9; the altar over the soul’s of the martyrs asking about judgement. This 8th angel offers incense at the altar, mixed with the prayers of the saints (Rev. 5:8), and the smoke rises up before God. Following this, the 8th angel takes fire from the altar and throws it onto the earth, resulting in fireworks and the sounding of the 7 trumpets.
So what does all this mean? Like much of Revelation there are many answers to that question. But it seems to me like a response to the prayer of the saints, in particular those under the altar asking about the coming judgement of their killers. After the prayers of the saints are offered to God, fire is poured out on the earth, and judgement begins.
Does God answer prayer? Yes he does; in his time. The martyrs had earlier been told that the judgement would come in the right time (Rev. 6:11), and that time had now come. Don’t be discouraged if your prayer is not answered when you think it should. Be content in knowing that if you pray according to his will (1 John 5:14), that his response will come in the appropriate time.