What does Ruth 3:2 mean?

As Naomi begins her instructions to Ruth, she identifies Boaz in two ways. The first is as "our relative." As such, he would be responsible to buy Naomi's husband's field from the man Elimelech sold it to so that the land would stay in the clan and Naomi would not be destitute (Leviticus 25:25–28, 47–49).

The second is as the man who took great pains to protect Ruth as she worked. Ruth has spent the last two months in Boaz's fields gleaning grain. This meant scavenging dropped stalks from the hired workers. Typically, a gleaner would make no more than a modern person today who collected aluminum cans from the street. But Boaz was so impressed by Ruth's desire to provide for her mother-in-law that he told his reapers to leave extra stalks of grain for Ruth to pick up. He also fed her, arranged for her to take water with his reapers, and told her to stay close to the women he'd hired to bundle the stalks together, thus keeping her safe from any man who was tempted to harm a foreign widow (Ruth 2:8–9, 14–16).

The passage doesn't explain why Naomi thinks Boaz will be winnowing his grain this night. It might be because it's sufficiently windy. Or, because he has been using the communal threshing floor and town gossip has revealed he has more to do.

Winnowing is one step in processing the harvested grain. First, the grain is threshed—crushed by foot, a sledge, animal hooves, or heavy stones—to break the husks away from the kernels. Then the grain is winnowed: tossed in the air so the wind can blow away the lighter husks, leaving the kernels. The threshing season is a celebratory time, especially after such a long famine (Ruth 1:1). It's also a time for eating, drinking, and immorality. Likely, Naomi mentions how Boaz protected Ruth to reassure them both that Boaz will not do anything to harm the young woman.