In this chapter, Ruth visits Boaz at the threshing floor and proposes to him.
This is another very unusual chapter. There are a couple minor cultural notes I will make, but the most significant thing in my opinion is how Ruth takes the initiative to go visit Boaz and in v. 9, she essentially proposes to him, saying "you are a close relative, cover me with your blanket". I would guess that's a euphemism for marriage and/or sex.
In many places in the bible, women are passively given in marriage. For example, Abraham sent his servant to go get Rebekah as a wife for Isaac. Later, Jacob bartered with Laban for Rachel and Leah. Rachel did not take any initiative to be married to Jacob, and as far as the story tells us, nobody even asked Rachel for her opinion about marrying Jacob. And that's just kinda how things went. For what it's worth, Laban did ask Rebekah if she wanted to go with Abraham's servant in Gen 24:58, but as a general principle, marriages were usually arranged.
That doesn't mean women were married against their will, but they weren't really the decision-maker. That role, as in the cases of Rebekah and Rachel, typically fell to their fathers or the senior men of the household. This was just another expression of the patriarchy that existed in Hebrew culture.
In this context, Ruth's actions are very striking because they defy social convention. Yet this was at Naomi's urging, and Boaz himself describes Ruth's actions as a "kindness" because she "did not go after young men". So I think Ruth's boldness is unusual, but it's perhaps even more surprising how favorably she is depicted by the author of this book. In Boaz's words, Ruth is a "woman of excellence". I believe that we can infer the author's opinion by these comments, which he chooses to include in the text.
I think that's the most interesting part of this chapter. There are a few other cultural inferences we can make. First of all, note that harvest time is typically a time of celebration in agricultural societies. It is the time of year when all of the hard work plowing and sowing and maintaining crops finally pays off, a time of great abundance when people stop having to live off last year's harvest and can eat from the new crop. That's why verse 3 talks about Boaz "eating and drinking" at the threshing floor, because it is more or less a party.
Even more allusively, apparently women are not permitted at these threshing floor parties. Naomi tells Ruth to go secretly, and Boaz himself warns her to "let it not be known that a woman came to the threshing floor". This is not something that is in the Law, and it's not mentioned anywhere else in the bible, so there isn't any additional context that I can bring to this. In the context of this story, it serves to highlight the boldness of Ruth's actions. Not only was Ruth trying to initiate a relationship with Boaz, but she did so while secretly going to a men-only celebration event.
Boaz must have liked her, because he agrees to marry her if he can, and also sends her away with six "measures" of barley (probably ephahs?). But first Boaz has to settle this closer relative who in the levirate system, has higher priority to marrying Ruth than Boaz does (v. 12-13). So Boaz needs this kinsman redeemer to relinquish his claim on Ruth before Boaz can marry her.