v1-5 - John cuts away from the scene in chapter 13 where the dragon and two beasts were deceiving the entire population whose names were not written in the Lamb's book of life to remind us, his readers, that no matter how successful the forces of evil seem, the Lamb still stands victorious.
He stands on Mt. Zion, that figurative mountain representing the City of God on earth along with the full number of the saints who've gone before, represented by the symbolic number of 144,000.
Having been purified in Christ, they sing a song only they can understand.
v6-12 - John tells of angels flying about, proclaiming in a loud voice the impending victory of Christ that will result in doom for His enemies.
v13-20 - The final 'harvest' of the earth, in which the elect are swept up into heaven and those who've rejected Christ are crushed like grapes in a winepress, will be short, swift and final.
The elect "will rest from their labor, for their deeds will follow them (v13)."
Those who've been marked by the beast "drink the wine of God’s fury (v10)."
Most people go through life stuck in a cycle of hopelessness because all they can see is the apparent victories of the dragon and beasts in this world. This is why it's so important for you to be reminded often of what the Holy Spirit enabled John to see and write down for the future generations of the church, so that you can have confidence that Jesus Christ has been, is now, and will be victorious over all things.
Verse 12 is a really good synopsis for the entire gospel, and especially what you are to remember the next time life seems dark and hope seems gone: