In regards to the investigation taking place in the play, the so-called "trifles" to which Mr. Hale refers to in the play would be the unimportant details that women presumably love to think about right in the middle of something bigger, more important, and more influential.
This is the view of the males in the play, anyway.
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 fact that Mrs. Hale and Mrs. Peters were concerned with Minnie Wright's preserves had less to do with the preserves, and more to do with them wanting to help a fellow woman comply with her duties as a housewife, which was her primary role. The men, however, saw it as a two women who were completely unconcerned with what really mattered.
The play continues in the same fashion, and following the same pattern where the women would divert their attention to a detail that goes unseen by the men only to realize that such detail is vital to the overall timeline of events of the fateful night when Minnie kills her husband.
- the disarrayed stitching- a "trifle" which clearly shows the state of mind of Minnie before the killing
- the empty canary cage- another "trifle" that entails that her only companion was no longer with her.
- the box where the dead canary was found- a "trifle" that points at the motive of the crime. John Wright wrung the bird's neck to spite Minnie, and she snapped after years of abuse.
- the messy state of the house- another symbol of Minnie's state of mind
To the men, these things were signs of bad housewifery. To the women, these seemingly small items meant exactly what they did...if one is smart enough to put them in context.