"Which is what?" is a phrase that doesn't make any sense if used by itself. It usually follows another remark, where "which" points to a noun or noun phrase in the previous sentence. Examples: - You say the problem is all because of one thing? Which is what? (What is that "one thing" you speak of?) - In everyone's life, there is a phase colored by a love, which is what gives it a greater meaning. ("which" = a phase colored by a love)