And THE NATIONS shall walk by its light,
The angel (verse 9) had directed our attention to new Jerusalem, THE CITY, and we became so involved that we have forgotten all about the new earth. Suddenly, John says that there are NATIONS. And these NATIONS shall walk or go about their business by the light of THE CITY. Considering how large THE CITY is (verse 16), it is understandable that a lot of light is given off especially since it is GLORY-OF-GOD powered (verse 11).
Now let's look at whom are THE NATIONS? Several commentaries point out that the root word here translated as NATIONS often is translated as 'Gentiles'. It is unclear whether 'Gentiles' here means non Jewish or non CITY residents. And where did these Gentiles come from? It is not told here what the origins of THE NATIONS might be. Purely by speculation, we could make an argument that the population at the end of the Millennial period who did not become deceived by the devil (Revelation 20:7,8) are translated to the new earth from their Millennial homes. Recall, the deceived people were burned (Revelation 20:9). These people are "saved" ones but not in the same sense as the BRIDE (the church) (verse 9).
and THE KINGS of the earth shall bring their glory into it.
The leadership among the populace of the new earth are here denoted as KINGS. It is not clear what 'their glory' might be. Perhaps, in a sense, we are seeing the garden of Eden all over again as it was BEFORE Adam and Eve fell for the devil's ploy (Genesis 2:4-15-25). We can assume that nakedness on the new earth will be OK too. As in the garden of Eden before the fall, there is nothing to be ashamed of in the new earth either (Revelation 3:18). It is left to the reader to determine what the opposite of ashamed is. Whatever that is, that is what THE KINGS bring into THE CITY. This verse calls it 'their glory'. There are clues in Revelation 4:8-11, Revelation 5:8-14, and Revelation 7:9-12-17.
One last thought: read Isaiah 60 and see how much of this verse you see in that chapter?