Mythologies in this case should not be construed as something untrue or pertaining to ancient history but rather as the stories we use to understand our daily life. Recognizing that our perceptions are shaped by language, Barthes pointed to the narratives which shape our views of the world. What is important about this analysis is that Barthes shows us that we take these myths to be natural. Myths hide nothing rather they present ideology at their face value. Barthes famous analysis of a black soldier saluting the French flag, or Stuart Hall's analysis of the black olympian draping the Union Jack around his shoulder during a victory lap are startling statements about the success of European imperialism.
Society is organized by codes of convention.
But it denies the reality of this code. Barthes talks about denotation and connotation to clear this issue up. The black soldier saluting the flag is just that at the denotative level. But at the connotative level its lends itself to rich historical interpretation which has much to say about French society and racism. Review the materials at Semiotics for Beginners by Daniel Chandler.