A place in hell called: The bottomless pit, where the dead [those who rejected the Living God] and the demons are kept in chains.
1. Then I saw an angel coming down from heaven holding the key to the abyss and a great chain in his hand.
It seems that this heavenly angel was in charge of keeping the demons locked up who had sinned in the time of Noah.
2 Peter 2:4-5
4 For if God didn’t spare the angels who sinned but cast them into hell and delivered them in chains of utter darkness to be kept for judgment; 5 and if he didn’t spare the ancient world, but protected Noah, a preacher of righteousness, and seven others, when he brought the flood on the world of the ungodly
2 He seized the dragon, that ancient serpent who is the devil and Satan, and bound him for a thousand years.
The dragon who is Satan was seized by the angel in charge of the abyss and locked him up with chains. For this angel to defeat the devil, reveals that he is a mighty angel of high ranking.
Thousand years refer to the earthly reign of Christ with His saints.
3 He threw him into the abyss, closed it, and put a seal on it so that he would no longer deceive the nations until the thousand years were completed. After that, he must be released for a short time.
Put a seal on it shows that the devil’s sentence was sure to be completed with no one to open and release him until the 1000 years was over.
Would no longer deceive the nations. No longer would the world have to deal with Satan’s deceptions and schemes but would be free from his devices so that the world may enjoy Jesus’ righteousness on the earth with peace.
4 Then I saw thrones, and people seated on them who were given authority to judge. I also saw the souls of those who had been beheaded because of their testimony about Jesus and because of the word of God, who had not worshiped the beast or his image, and who had not accepted the mark on their foreheads or their hands. They came to life and reigned with Christ for a thousand years.
Then I saw thrones means that the saints will have a role in the millennial reign of Christ. They will judge the nations all over the world.
The saints who refused to accept the mark of the beast in the Great Tribulation and were killed, seems to be given a special position with Christ in His millennial reign.
If a person receives the mark of the beast on their hand or forehead, they will not reign with Christ but will be cast into the Lake of Fire [Revelation 14].
5 The rest of the dead did not come to life until the thousand years were completed. This is the first resurrection.
The rest of the dead did not come to life refers to the lost, those who rejected Jesus Christ as their Savior.
Only the saints will participate in the first resurrection and reign with Christ.