it means 'important'. It's a colloquialism. "They just made me president of the university." "Whoa, congratulations! That's a big deal!" But often it is used sarcastically -- which might be the confusing part -- in which case it MOCKS the idea of something being important. "They just made me president of the university." "Big deal. It's a lousy job. They don't pay you and no one listens to you."

It depends a bit on the context. It can mean important, well-known, famous, trendy...