It's from one of the "Aesop's Fables". A fox sees a bunch of grapes hanging a ways over his head. He spends a while jumping and trying his best to reach them, then gives up and stalks away muttering "they're obviously sour".
"Driven by hunger, a fox tried to reach some grapes hanging high on the vine but was unable to, although he leaped with all his strength. As he went away, the fox remarked 'Oh, you aren't even ripe yet! I don't need any sour grapes.' People who speak disparagingly of things that they cannot attain would do well to apply this story to themselves."
Source: https://en.wikipedia.org/wiki/The_Fox_and_the_Grapes
At their best, they seem to describe timeless patterns in human behavior. http://read.gov/aesop/001.html
(note, the library of congress version has the 'moral' spelled out at the end, in a way that sucks the life out of what you just read)