Revelation is a vision given to the disciple John and is written in apocalyptic style, a style rich in imagery and metaphors. Revelations does not just address the end times but John also addresses the churches of his time, encouraging to repent of their ways and to keep their faithfulness to God during the times of persecution the church was facing. John was writing this on the island of Patmos because he was exiled from the Roman Empire.
The New Jerusalem descending from the sky is the restoration of God's original plan. With sin and evil cast out of the world God can once again dwell amongst His people, providing everything for them. Just as God walked in the Garden with Adam and Eve He will walk amongst us again in the city of eternal life.
With evil and sin out of the world there is also the end to death, sorrow, or pain (all consequences of sin). God will wipe every tear from our eyes and comfort us.
The city itself is a bright, shining city of precious jewels and gold. A city illuminated by the glory and truth of God. A river of life flows through the city coming from the throne of Jesus. The city of light reflecting Jesus, the light of the world (John 8:12), and the river of life coming from Jesus, so those who drink of it may never thirst (John 4:14).