what's poppin' fresh?

This is a greeting meant to say what's up, what's crackin', or another such colloquial greeting. The reason it is popular is because Poppin' Fresh is the Pillsbury Doughboy's former name (the name was changed to update little Freshy's character and bring him into modern times). The phrase is not questioning what Poppin' Fresh is, rather it is using poppin' in it's slang meaning, and it is using fresh as a noun, not an adjective. Fresh refers to the person being addressed by the question.