There isn't as much information about this story out there as some of O. Henry's more well-known works, but if you type the title into google you can get some pretty interesting hits and discussions about the story. I supplied some information below, to get you started.
This story of O. Henry's is a pretty sad one; it starts optimistically, with a woman who works at a bakery taking notice of a man who comes in a couple times a week to get stale bread. She starts thinking about him, romanticizing things, and wonders why he is so poor and only gets stale bread. She feels sorry for him. One day, in an attempt to be generous, she sneaks some butter into the stale loaves, only to have him come back, furious. A younger man lets her know that he used the stale bread as an eraser; he was an architectural draftsman, and apparently, stale bread worked better than rubber for an eraser. So, the butter had ruined all of his drawings, and he had been ruined as a result. Pretty depressing story, huh? The story itself is set in a city, probably about 100 years ago or so (it never states what time period, but one can guess from the bakery, and from the man having a German accent that it might be a time when European immigrants came to America in large crowds seeking work-about 100 years ago). The only two characters really are Miss Martha Meacham and the German man, who she assumes is an artist (until the end). The style is rather straightforward, humorous, and a bit sarcastic, as a lot of O. Henry's works are, and the theme would be something along the lines of never assuming anything about anyone, don't be judgmental, or how presumptions can be wrong and dangerous.
I hope that helps a bit; good luck!