Common knowledge is something considered to be known by a large group, if not all, people. Common knowledge could be of what a cake is, or the story of little red riding hood. It is considered a fact that you can confidently state and that many people will be able to relate back to.
Assuming this is when they first meet, Boo is different from what they think because he is not the mean dangerous man he is rumored to be but rather kind and sweet.
