What does "sour grapes" mean?

Disparaging what one cannot obtain, as in The losers' scorn for the award is pure sour grapes. This expression alludes to the Greek writer Aesop's famous fable about a fox that cannot reach some grapes on a high vine and announces that they are sour. In English the fable was first recorded in William Caxton's 1484 translation, “The fox said these raisins be sour.”