If you got a fortune cookie with the message "Your future will contain many bananas," you might ask, "What on earth does that mean?" In other words, you wonder what the intention or meaning of the fortune might be.
Mean is one of those ordinary, everyday words with lots of different meanings. The adjective mean can describe someone who's stingy or ungenerous, but it also means "unkind or unfair," which is what a little kid intends to convey when she yells at her mother, "You're mean!" In the sentence, "She lived in a mean little house," mean has yet another meaning, this time being "shabby or poor."