It means "Are you finished?" or "Is it time?"

Comes from a 1970's Lipton instant soup commercial, notable on several levels. First off, it was a slam against Campbell's which was merely condensed. Lipton did Campbell's one better and removed itself even further from the real stuff, by removing ALL the water and powdering the soup. Only when re-hydrated with boiling water, was it again soup.

So kid's in commercials would ask, "Is it soup yet?"

AND BACK THEN, THIS WAS SUPPOSED TO BE A GOOD THING!!

The fact that it was even more prefab than Campbell's was a selling point!

When you eat dirt for the first time and leave your family to build a shack in the woods