It's a common expression used in everyday conversation, and it can be used in a variety of situations, such as when you:
* Hear unexpected news
* Witness something surprising
* Are impressed by something
Here are some examples:
* "Ça alors, tu as gagné au loto ?" (Well, I'll be, you won the lottery?)
* "Ça alors, c'est magnifique !" (Goodness me, that's beautiful!)
* "Ça alors, je n'aurais jamais pensé que tu ferais ça !" (Well, I never, I would never have thought you would do that!)