How is Trifles an illustration of in medias res?
In Media Res is Latin for "In the middle of things" (Wikipedia) and is a common technique for starting a story with important events already past. This allows the author to avoid pages of explanation and exposition and explain the story by how the characters react to their current situation. Trifles is a good example of in media res as it starts with the characters gathering in the house of John Wright after his death; the beginning of the play is the beginning of the investigation.
SHERIFF: [Unbuttoning his overcoat and stepping away from the stove as if to mark the beginning of official business.] Now, Mr. Hale, before we move things about, you explain to Mr. Henderson just what you saw when you came here yesterday morning.
COUNTY ATTORNEY: By the way, has anything been moved? Are things just as you left them yesterday?
SHERIFF: [Looking about.] It's just the same.
(Glaspell, Trifles, etext.virginia.edu)
With the important event of Wright's death out of the way, the characters can talk about what they think happened, informing the audience while moving the story forward. The story hinges on the empathic understanding of the women while the men look for a more simplistic motive; if the audience had witnessed Wright and Minnie's home life before the murder, they would already know the ending before the more dramatic reveal. Therefore, Glaspell started the story "in the middle of things" to keep the audience guessing, just as the characters are.