Your son had a bottle of juice. He drank it all. You hold up the bottle and show him that there's no more left.
When there's no more of something, you can say that it's "gone":
A: Where's the coffee?
B: It's gone.
"All gone" is the same, but it sounds a little more playful. You use this phrase when speaking with young children. For example, you can play a game with a baby where you show them a toy, then put it behind your back and say this: