Myth, according to Roland Barthes in 'Myth Today" (in Mythologies) does not reside in the simple denotational meaning of signs. With myth being a secondary system of signification in which the sign becomes a signifier, that process of signification takes the meaning of symbols in the myth to the realm of association, not denotation.
According to Barthes myth, as a form of speech, is not limited to lingual signs and other types representation (visual, musical etc.) can also take part in a myth because they convey secondary meanings that surpass their referential denotation. Bathes gives the example of a magazine cover portraying a African child in uniform saluting the French flag. The first level of signification is the denotation one – the child saluting the flag. But in the second level of signification, that of myth and connotation, the sign becomes a signifier and the child hails France as a great empire.
Since myths do not convey their meaning directly but rather in a covert manner, Barthes calls his semiology of myths "a science of forms". In the picture Barthes analyzes everything works together, the child, uniform, flag, salutation etc. to produce the desired meaning and to establish the myth. This is what according to Barthes distinguishes his concept of myth from the Marxist concept of ideology, since the science of myth is engaged with the expression of meaning through formal means.
For Barthes, meanings and myth are historically produced and conditioned, and they are not eternal but rather constantly mutating and reforming. This means, under Barthes' Marxist perception, that myths are always political in being the result of specific power structures in a certain society at a certain time. But myth his the capacity to disguise its own historicity and to present itself as objective and natural (Althusser noted that ideology has not history). What myth does, according to Barthes, is to de-historicize and de-politicize meanings that are always historical and always political (similar notions can be found in Benjamin's "Art in the Age of Mechanical Production").
Roland Barthes - Myth Today - Summary, Analysis and review - part 1 - part 2 - part 3 -criticism
See also: Roland Barthes - The Death of the Author