In this chapter, Jerusalem is sacked and all of the leaders of the city are exiled to Babylon.
There are several broad patterns we can draw out of this chapter. The central point is that Judah is getting systematically torn to pieces, but since we have to read a whole chapter about it, I'm going to write in more detail.
The first pattern is: foreign kings renaming the rulers of Judah. This first happened in the previous chapter when Pharaoh took Eliakim, made him king, and renamed him Jehoiakim (2 Kings 23:34). The second time it happens is in this chapter in v. 17 when Nebuchadnezzar changed Mattaniah to Zedekiah and made king. It might not be immediately obvious why Pharaoh and Nebuchadnezzar would do this, but this is an expression of domination. What is a more thorough way to change someone's personal identity than to change their name? God changes the name of several people in history (for instance, Abram renamed to Abraham and Jacob renamed to Israel). In those cases, God would rename people as an expression of his promises for them and their destiny and future. In this case, Nebuchadnezzar is renaming the king of Judah in order to show that the destiny of Judah and Judah's rulers is in the hands of Babylon. If Babylon can change Mattaniah's name, what could they not change?
The actual meaning of Zedekiah and Jehoiakim is godly ("the LORD is righteousness" and "the LORD raised him up" respectively), but the point is that this is not something that was done by God for their good; it was done by a foreign king to break their identity as a means of control.
The second pattern is the removal of leaders and authorities. This is described in verses 10-16 and it's done for two main reasons I can think of. First (and perhaps most importantly) it undermines the possibility of any resistance by removing the people most capable of organizing resistance. The leading men, the "mighty men". Why would Nebuchadnezzar take away all the best soldiers? It could be he wants to place them in his own army, but I think it's more likely because he doesn't want these commanders organizing the army of Judah to fight against him if the people decide to rebel (as they literally did earlier in this chapter, as described in v. 1). The second reason why Nebuchadnezzar would take all the best people, particularly the craftsmen and smiths, is to enslave them for his own profit. These are the people with the most skills and knowledge and the most earning potential. Nebuchadnezzar probably hauled them off to his own workshops to make things for his treasury or whatever other projects he had going on.
Lastly, Nebuchadnezzar removes the material wealth from the country. He's certainly not the first king to pillage Jerusalem's temple and royal palace (for one earlier example, 1 Kings 14:26), but combined with the other actions described in this chapter, it further drives Judah into a state of helplessness and paralysis. Without gold or silver in the royal treasury, the new king of Judah will have less financial resources to 1) raise an army and 2) pay for foreign mercenaries. Besides enriching Nebuchadnezzar, this will also make it more difficult for Judah to resist him in the future.
In conclusion, Judah is basically screwed. Also, there are bands of Moabite, Ammonite and other raiders traversing the land pillaging and murdering people. So that's also happening.
Why does this happen? The author takes yet another opportunity to remind us of the sins of Manasseh; interestingly, it's not the idolatry that seems to drive retribution against Judah, it is the innocent blood shed by Manasseh that has sealed their fate to destruction.
As another brief aside, I should also mention that even though the book of Kings does not describe the battle of Carchemish, we can see the results of it in this chapter where "the king of Egypt did not come out of his land again" (v. 7). The Egyptians, along with the Assyrians, were soundly defeated by the Babylonians. In the previous chapter, Josiah was killed in battle with the Egyptians when the Egyptians were rushing out to get to that battle and Judah resisted them. Now the Egyptians have been defeated and ironically it is the Babylonians that are now oppressing Judah.
Judah started off this chapter in a more dignified kind of servitude to the Babylonians, but there are not many nations that are happy with "dignified servitude", so they resist and are crushed. At the end of this chapter, we learn that Zedekiah now also rebels against the Babylonians and in the next chapter we will learn if he fares better than Jehoiakim.