What does "apostrophe s" stand for in "What's it all mean"?
As the chart of English contractions on Wikipedia points out, 's can be used in place of does (as well as is, has, was and as).
It can also be used to represent the entirely unrelated us, as in let's.
You're quite right. I've never thought about it before, or seen it listed, but it is quite common in "What's he do?"
It's not that surprising if you think about it, because the "d" of "does" gets assimilated into the "t" of "what", so it becomes "whattəz" or "whadəz"