"Trifles" means small things, insignificant details. While it is these "trifles" that give away Minnie's guilt, the women are the only ones who pick up on them. They put themselves in her shoes as any one of them might have done the same thing had they been in her situation. They recall the Minnie they knew before her marriage and all see what being married to such a cold man and isolated in such an unforgiving setting have done to the once happy girl who sang.
The women are her peers...not the men who are actually there to gather evidence against her. The women make a decision collectively to hide the things that would have convicted Minnie.
The reader wonders whether or not the men would have even discovered the evidence which condemns Minnie. They didn't take time to notice anything other than her unkept household. If the women had not been there to pick up a few things for her, perhaps Minnie's story would never have been told. They are "trifles" to the men, but they are the every day misery which tell the story of Minnie's neglectful and demoralising existence.
Because the "jury" judging Minnie Wright (Foster) have acquitted their peer of murder. The women, her neighbors and friends, understand that Minnie has been overworked, abused, and neglected.
Glaspell's overarching theme is that women's work is anything but "trifling"; without it, men, women and children could not survive. It is women who do the canning so that all may eat in the winter, women who cook the meals and clean the home and see to it that everyone has what they need. Yet when the men come to investigate the crime, all they see are Minnie's "trifles" and her messy kitchen. They have no appreciation for what Minnie has endured. But the women do.