Selected commentary on Revelation 7, 8 and 9 adapted from David Guzik at Blue Letter Bible.
Who are the 144,000? 
Characteristics:  The “children of Israel” (7:4), protected and triumphant through the period of God’s wrath, meeting with Jesus at Mount Zion at His return (14:1), celibate and “the beginning of a greater harvest” (14:4), marked by integrity and faithfulness (14:5).  It is best to see the 144,000 as specifically chosen Jewish believers in Jesus, protectively sealed throughout the tribulation as the beginning of the harvest of the salvation of Israel (Ro. 11:26, Mt. 23:37-39). 

v. 9 – salvation is God’s to give, not ours to earn.  

v. 13 – The presence of so many tribulation saints is a powerful statement of God’s grace and mercy. Even in this time of judgment and wrath on the earth, many are saved.  They are possible martyrs, dressed like the “souls under the altar”, whose number is now complete (6:11)

v. 14 – only the blood of the Lamb could clean their robes – Though your sins are like scarlet, they shall be as white as snow; though they are red like crimson, they shall be as wool. (Isaiah 1:18)

v. 1this silence in heaven demonstrates a sober, awestruck silence at the judgments to come, now that the seals are off and the scroll can be opened.
v. 2 – seven trumpets will sound as God’s battle-alarm during the great tribulation (OT ref.)
v. 3-4 Prayer and incense are often associated in the Bible. The idea is that just as incense is precious, pleasant, and drifts to heaven, so do our prayers. So here, before anything happens at the opening of the seventh seal, the prayers of God’s people come before the Lord God.
The first 4 trumpets reveal the severity of God’s judgment.  He attacks all the ordinary means of subsistence, such as food and water; and He attacks all the ordinary means of comfort, and knowledge, such as light and the regular rhythm of days.  We know the great humility that comes upon men in the midst of something like an earthquake, because they know that “nature” is not as reliable as they had thought.  The first four trumpets also reveal the mercy of God’s judgment; these are partial judgments striking only one-third, and are meant to warn and lead a rebellious world to repentance before the final curtain. For now, God spares more than He smites.
v. 7 – God may use whatever method He desires to bring judgment, but people on earth know these events are from God, and do not think them to be merely natural disasters (Revelation 16:9; 16:11; 19:19).
v. 8-9 – some literal mass of land, probably a meteor or asteroid, falling into the sea and bringing ecological disaster.
v. 10-11 – possibly a comet or meteor crashing into the earth and bring ecological disaster; Wormwood is a very bitter substance, and proverbial for bitterness and sadness.
v. 12 – This does not describe a one-third lessening of light, but one-third of the day and night are plunged into absolute darkness. As Jesus said: the sun will be darkened, and the moon will not give its light (Matthew 24:29).
v. 1 – the “star” is a person (“he” in v. 2), best seen as an angel; whether he is a good or bad angel depends on his relation to the angel of the bottomless pit in Revelation 9:11. If the angel of Revelation 9:1 one is the same as the angel of Revelation 9:11, it is an evil angel – perhaps Satan himself. If it is a different angel, it may be a good angel sent by God to open up this bottomless pit for the purposes of judgment.
v. 2 – “the Abyss” = bottomless pit = considered the realm of the dead, the same as Hades, a prison for certain demons (Luke 8:31; 2 Peter 2:4, and Jude 1:6).
v. 3 – as part of the judgment of the great tribulation, God will allow demonic hordes, previously imprisoned, to descend upon the earth like a swarm of destructive locusts.  Locusts are agents of God’s judgment in the OT.

v. 4 – Those who have the seal of God on their foreheads (the 144,000 and perhaps more) are protected, but none other are. This is an inescapable judgment of God

v. 5 – Their purpose and period is expressly governed by God, and the purpose of all this to bring repentance
v. 6-10 – All we can know for sure is that in the period immediately before the end the wicked will be subjected to a time of unprecedented demonic torment
v. 11 – Abaddon and Apollyon both have the same thought of destruction or torment; Satan himself or another high-ranking leader of demons.
v. 13 – In the tabernacle and temple of Israel, the golden altar was the altar of incense, which was a representation of the prayers of God’s people.  Atoning blood was applied to the horns. From these horns, John hears a voice. In this, John recalls a persistent theme: the prayers of God’s people play a large role in the end-times drama.  

v. 14 – The Euphrates was a landmark of ancient Babylon. It was the frontier of the Israel’s land as fully promised by God (Genesis 15:17-21). It was also the boundary of the old Roman Empire, which will be revived under the Antichrist.  It was also associated with the first sin (Genesis 2:10-14), the first murder (Genesis 4:16), the first organized revolt against God (Genesis 11:1-9), the first war confederation (Genesis 14:1), and the first dictatorship (Genesis 10:8-10).
v. 16 – the army – possibly human soldiers, but more likely demonic troops, flooding the Earth.  
v. 20 – 21 – we are so quick to forget God’s lessons, even the lessons that come in judgment.