A Greek word meaning "emotion," pathos in literature tries to tug at our heartstrings, big time. That is, we're supposed to really feel something when we read pathetic literature. If an author plays the pathos card, we're not just vaguely bummed, we actually emote pity, sympathy, or straight-up sorrow.
Want more? Think of the way Dante creates sympathy for the denizens of hell (an impressive feat!) in Inferno or the way Mark Twain makes you feel pretty bad for an old drunk criminal named Muff in The Adventures of Tom Sawyer. If a book makes you feel kind of sad for drunk criminals or major sinners, then you know there's some serious pathos at work.