Like any self-mythologising French philosopher, Barthes doesn't make life easy for the reader. He's not as nonsensical as Baudrillard, but then I think he preceded him, giving the latter the chance to up the stakes still further.
Anyway, Mythologies is about myth, which is what Barthes calls second- or further-order semiology - when something that's already been used to signify something is then used in another context to signify something else - and what happens to the original thing and what it originally signified, as well as the properties of what those two together then jointly signify at the new level. Clear? It won't be.
But there are some fun examples, like what beads of sweat mythologise in the film Julius Caesar, and what a photo of a writer on holiday mythologises. It's good stuff in places, I just wish it was less deliberately opaque in others.