At an American wedding, a traditional poem is read, "Something old, something new, something borrowed, something blue." How do you make sense of that?

It's custom. The custom says that you need an old thing, a new thing, a borrowed thing, and a blue thing to get married, and it is said that this is the only way to have a happy 100 years of marriage. Hong Kong writer Yishu also mentioned "a little new, a little old, borrowed and blue" in her works, also based on this allusion.