Usually it means the fashionable people who are aware of and follow things like music and entertainment. People who dress well and look good. People who know all the newest cultural references.
Restaurants cultivate a certain demographic, a category of society. If the restaurant goes for the trendy crowd they tend to be selective about who they let in, just as the trendy people are selective about where they go. If the restaurant "attracts" trendy people it is because there is a mutual understanding between the place and the people that each considers the other to be worthy of themselves. I hope that's as clear as mud because it certainly is to me.