"Pie in the sky" is an idiom, similar to "castle in the air" or "jam tomorrow". It refers to a fanciful idea, unrealistic goal, or desired outcome that would be nice to obtain but is unlikely to happen.
"One day I'm going to be a rock star!" "Don't you think that's a bit pie in the sky?"
"100 nodes is my pie-in-the-sky goal for November, but I expect I should be able to write at least 30 for the Iron Noder challenge."
"Pie in the sky" originally meant a heavenly reward after death. Joe Hill wrote a song in 1911 called The Preacher and the Slave, which was a parody of a hymn and was meant to criticise the Salvation Army, who were focusing on saving souls and were ignoring earthly needs such as hunger.
You will eat, bye and bye,
In that glorious land above the sky;
Work and pray, live on hay,
You’ll get pie in the sky when you die.
The phrase gained popularity in World War II and also took on its current usage of meaning the unlikely prospect of achieving a goal or future happiness.