The play concerns the small things in life that build up and cause distress and anger. The men, who are not considering the possibility that Minnie murdered her husband because of his abuse, ignore the signs of unhappiness they see in the house, but the women notice and interpret those signs correctly.
SHERIFF: Well, can you beat the women! Held for murder and worryin' about her preserves.
COUNTY ATTORNEY: I guess before we're through she may have something more serious than preserves to worry about.
HALE: Well, women are used to worrying over trifles. [The two women move a little closer together.]
(Glaspell, Trifles, etext.virginia.edu)
The men think that the women are only worried about "trifles," or inconsequential things. They assume that the sewing, the preserves, the empty birdcage are all just things, without meaning, and so ignore the evidence that the other women correctly interpret: Minnie was desperately unhappy and wanted to escape, but felt herself unable to escape on her own. The death of her canary was the last straw; the men think nothing of the loss of a canary, but to Minnie, the canary was a vitally important part of her personal identity. A "trifle" to some, to her it was the last thing that made her life worth living.