In this chapter, David fights his last war against the Ammonites and his men slay some giants.
This is the last chapter describing David's battles in the book of Chronicles. The contents of this chapter are copied from two places in Samuel, 2 Samuel 12 and 2 Samuel 21 with little modification (a few details are left out from Chronicles, but I don't think it's a substantive change).
From the original Samuel narrative, the story of David's battle against the Ammonites was part of the central narrative, right after David sinned with Bathsheba and killed Uriah. The story of David's men fighting against the giants is taken from the end of the book in what forms a kind of appendix to Samuel. The last four chapters of Samuel (chapters 21-24) are an appendix to the book. They contain a series of somewhat disjointed stories (and a few songs) taken from David's life. They don't really fit into the primary narrative and don't really relate to each other. I think they are perhaps just moralistic vignettes; short stories intended to convey some "point" or other, or perhaps just an illustration of David's character.
In any case, since the Chronicler is copying from the appendix here (and in the next chapter also) he is signifying that this is the end of the Samuel-based narrative. In fact, the Chronicler does not copy any more of the story from Samuel. From this point on, Chronicles begins its own narrative describing the preparations for the tabernacle, appointing officials and Levites, and many similar things. Just as important as what the Chronicler adds is what the Chronicler takes away: 2 Samuel 13-20 disappears entirely from this story. These are the "dark years" for David, when he faces the rape of Tamar, the murder of Amnon, and the rebellions of Absalom and Sheba, and Chronicles doesn't want to cover any of it. Basically, the story covers the part of David's life when he is ascending as king, recovers the ark of the covenant, conquers Jerusalem, and defeats all his enemies. The many years when David hid from a psychopathic king Saul or fought a civil war against Saul's heir Ish-Bosheth are left out, and the many years when David finds himself battling his sons are also left out. Chronicles also skips over the earlier part of chapter 12 that describes David's sin with Bathsheba, which shows that this curation is intentional. In the middle we are left with David's golden years as the sole focus for this book.
In its place, the Chronicler adds many unique passages describing the preparations for the temple. From this, we can make two observations: first, Chronicles is not intended to be a narrative about David. Second, Chronicles is mostly a narrative about the temple. David is relevant because of his relation to the temple. On the other hand, Samuel is a narrative about the history of the kingdom, so it remains focused on the sequence of kings that reigned from Saul down through Solomon, continuing smoothly with the book of Kings.
With all that said, this chapter fulfills a very similar role to the previous two chapters, establishing peace for Israel and bringing in the plunder that is used to finance the temple. It also shows that after David started killing giants, that he was able to train and inspire his men to also kill giants which is the mark of a great leader. A great soldier can kill giants, a great leader inspires his followers to kill giants.
In the next chapter, the narrative copied from Samuel concludes with David demanding a census and a plague striking Israel.