The title is almost deceptively simple. A man's name. It's kind of an odd name though. Ethan Frome. There is something solid and immoveable about it. And something old-fashioned. This novella is all about Ethan, or, more accurately, it's about the narrator's vision of Ethan, and his adaptation of Ethan's tragic story. The novella is split between the narrator's real interaction with the older Ethan, and his story of the story of the young Ethan. And by story, we mean the story that the narrator is able to imagine after spending the night in the Frome house.
Chapters 1 through 9 can be seen as an exercise in imagination. The narrator makes an imaginative study of Ethan, an extraordinary and mysterious man who has been badly damaged. The title tells us that Ethan is probably the muse of this story, the spark that lit the narrator's creative fire.
This is not to say that Mattie's story and Zeena's story are less important, or less tragic than Ethan's. In fact, the title lets us know that not all the characters will be given equal attention, and that we'll have to read between the lines to get at their sides of the story.