What are the “trifles” to which the title refers? How do these “trifles” advance the play’s plot?

“Trifles” are defined as things which have no value and are considered to be insignificant. Ironically, the men who are so determined to find convicting evidence look past the observations of their wives because they're seen as insignificant trifles, not realizing that the "trifles" were exactly what they needed in terms of evidence.