Context Summary
Revelation 21:1–8 continues the progression of events which came after the end of the tribulation: Christ’s return to earth (Revelation 19:11–16), the defeat and destruction of those who war against Christ (Revelation 19:17–21), the incarceration of Satan (Revelation 20:1–3), the millennial reign of Christ (Revelation 20:4–6), the release of Satan and the nations’ final revolt against God (Revelation 20:7–10), and the great white throne judgment (Revelation 20:11–15). Here we see the creation of the new heaven and the new earth. Upcoming verses describe the New Jerusalem (Revelation 21:9–27).

For a more detailed summary select the link below.

“Revelation 21:1” Bibleref.com

What does Revelation 21:1 mean? | BibleRef.com

Revelation 21:1

I saw a new heaven and a new earth: for the first heaven and the first earth have passed away, and the sea is no more.