A closed minded. person is often given to people who are very firm I'm there own beliefs. Take for example a person who is against gay people because they believe it is "unnatural." Now, it is very common for some people to express there feelings towards others about their beliefs; good or bad.
so this quote is saying, that people who are closed minded( ex: white supremists, homophobes, feminist, etc) are often the ones who complain the loudest. the author of the quote is saying that these types of people shouldn't say anything, as everything they say is either offenseive or just plain dumb.
I hope this helped a little, its a bit hard to explain without using examples.
