It's used to ask someone "what do you think about this?" It is certainly not as common as "What do you say?" or "What do you think?" or "What is your opinion?" -- but it is idiomatic English. It is old-fashioned and appears mostly in spoken English these days.
It has a slightly urgent but familiar tone:
What say you, Mr. Brown? Will we have an early frost this year?
I think she should take a semester off before returning to college. What say you?
The expression can also be a slightly aggressive way to ask a question. In this context, "what say you?" means "what do you say in response?" Here's a couple of examples:
You made your choice, but what say you to young people who struggle with that dilemma even as we speak?
You've heard all the evidence. What say you to that?
It is also part of the old-fashioned and more formal language of courts of law, and is used to ask about decisions or to ask a defendant to issue an official plea of "guilty" or "not guilty":
To the charge of murder in the first degree, what say you?
What say you, foreperson? Is the defendant guilty or not guilty?
[Thanks to editor Kory Stamper for some of this information.]