Some translations set each line of poetry farther to the right than the rest of the text to make it easier to read. The ULB does this with the poetry in 21:1-10, 13-17.
This is a metaphor for an unknown nation. Scholars are divided over the proper identity of this nation, but it is probably Babylon. (See: [[rc://en/ta/man/translate/figs-metaphor]])
The fulfillment of this chapter is very mysterious. It is unknown who is truly being addressed and how this is to be fulfilled in history. The translator should not attempt to give the reader insight into this prophecy's fulfillment. (See: [[rc://en/tw/dict/bible/kt/fulfill]])