The meal, especially dinner, is served. Refers to any food being served, not just soup. OK, gang, soup's on! Please wash your hands and come to the table! A: "Soup's on, Bill!" B: "OK, honey. I'll be there in two minutes."
See also: on
Rur. The meal is ready to eat. (Said for any food, not just soup.) Tom: Soup's on! Bill: The camp chef has dished up another disaster. John: soup's on! Come and get it! Mary: Well, I guess it's time to eat again.