The title Trifles refers to the dismissive way the men investigating the case of John Wright's murder treat all the bits of evidence that would have led them to solve the crime had they paid attention to them.
Early on, when Mrs. Peters wonders over Minnie Wright letting the kitchen get cold enough to freeze her preserves so that they later explode, the sheriff says,
Well, can you beat the women! Held for murder and worryin' about her preserves.
Mr. Hale responds,
Well, women are used to worrying over trifles.
The "trifles" or details that the men laugh at are, however, important keys to solving the crime. Ironically, they are not trifles at all. Because Mrs. Hale and Mrs. Peters know what a woman's life is like, they are able to discern which details are important to reconstructing how and why Mrs. Wright killed her husband.
These "trifles" include the messy state of the kitchen, which the men dismiss as evidence that Mrs. Wright was a poor housekeeper. The women focus, too, on the broken door to the bird cage and then find the body of the dead canary with its neck broken. Even the sudden erratic quality of Mrs. Wright's stitching becomes an important clue to the women, who know how to read the evidence.
The title points to a main theme of the play: that men are blinded by their own arrogance when they refuse to take the details of women's lives seriously.