On the wikipedia for post structuralism,
In Elements of Semiology (1967), Barthes advances the concept of the metalanguage, a systematized way of talking about concepts like meaning and grammar beyond the constraints of a traditional (first-order) language; in a metalanguage, symbols replace words and phrases. Insofar as one metalanguage is required for one explanation of the first-order language, another may be required, so metalanguages may actually replace first-order languages. Barthes exposes how this structuralist system is regressive; orders of language rely upon a metalanguage by which it is explained, and therefore deconstruction itself is in danger of becoming a metalanguage, thus exposing all languages and discourse to scrutiny. Barthes' other works contributed deconstructive theories about texts.
The idea of symbolic logic existed already in 18th century (1700s), so what is new in Barthes ideas?
Note: As far as I understand this meta language idea is the same which underlies propositional logic/predicate logic. See here for an exaplanation given in a mathematical Logic book Stephen Kleene ( Student of Alonzo Church).