The title is one of my favorite Peter Drucker quotes. What he is referring to is the fact that common wisdom (what the majority believes is true) is often false. Take for example, that the majority of Americans believe in paranormal phenomena, when there has been no scientific evidence to support it.
The point is that we need to be critical thinkers and search for evidence or data to support assertions before we simply believe what the majority believes. This is very important in an election year, because candidates have been throwing out common beliefs as facts. Take for example, the issue about who pays taxes ("51% of Americans pay no taxes" = False), and misconceptions about who are the recipients of government assistance programs.
This morning I came across a very interesting article about U.S. economic beliefs, entitled "3 Economic Misconceptions That Need to Die." The misconceptions are: (1) that Americans spend the majority of their money on goods made in China (it's only 2.7% of our expenditures); (2) China holds most of U.S. debt (they own 7.6%), and (3) most of our oil comes from the Middle East (only 9.8%).
When it comes to leadership, there are also common misconceptions, such as the belief that leaders are born, not made (answer: more made than born; good thing since we spend billions on leader development).
The point is that with the Internet it is easy to do your own research. Don't take statements and "facts" at face value, because what people believe is true is frequently wrong.