In To Kill a Mockingbird, to what did Mr. B.B. Underwood compare Tom's conviction and death?

The passage that you are looking for is a couple pages into chapter 25, and are key passages that relate to the theme and title of the novel.  B.B. Underwood writes about Tom's death and conviction in the newspaper, and he "was at his most bitter" about the events that occurred.  He writes an editorial, which is usually a piece of writing that expresses your opinion on important topics of the day.  Newspapers like to feature editorials on current events, that express differing viewpoints on issues.  Underwood, in his editorial,

"likened Tom's death to the senseless slaughter of songbirds by hunters and children."

He basically compares Tom to an innocent songbird that is going about living his life, bringing happiness to his family and working hard, that is shot down for sport, cruelty or pleasure by other people.  It is in this passage, connected with a couple other passages where Atticus says it is a sin to kill a mockingbird, that the title of the story comes from.  Tom is compared to a mockingbird that is senselessly killed by the town.  Mr. B.B. Underwood is the one to clarify and bring out this point, and it is a major theme throughout the novel, a theme of the innocent suffering because of other people's cruelty and prejudices.  I hope that helps a bit; good luck!