Although not all Christian sects agree on the nature of the New Jerusalem, this city is a major part of Christian eschatology. The New Jerusalem is, essentially, heaven.
If the book of Revelation is to be taken literally, the New Jerusalem will be a city that is a perfect cube with dimensions of about 1400 miles on each side. The city is to be made of all sorts of precious stones and of gold. Most importantly, the New Jerusalem is to be a place in which the saved will live with God. He will actually live with "his people." In this city there will be no day and night because the glory of God will provide constant light forever.
In a nutshell, then, the New Jerusalem is heaven come down to Earth.