As a stand alone question, it could be closely translated as an informal way to say "What is that?"
A: "This weekend, I got an Akita."
B: "Which is what?"
A: "It's a breed of dog."
The general idea is that person "B" is asking the question by implying person "A" should have explained further themselves. Because person "B" doesn't know what an Akita is, they replace "a breed of dog" with the iterrogative pronoun "what."
Note that this is informal and may seem a little rude to the other person.