Common knowledge is information that everyone knows and, importantly, that everyone knows that everyone knows, and so on, ad infinitum. If information is common knowledge in a group of people, that information that can be relied and acted upon with the trust that everyone else is also coordinating around that information. This stands, in contrast, to merely publicly known information where one person cannot be sure that another person knows the information, or that another person knows that they know the information. Establishing true common knowledge is, in fact, rather hard.
See also: Public discourse