This is the diagram that Barthes devised to show the relationship between language and myth. The first-order language relationship between signifier and signified is shown on the top row. The first-order sign labeled [3. Sign] then becomes a second-order signifier in the second row. This second row is where Myth draws on language to build its meaning system.
Rows 1 and 2 in Barthes diagram refer to LANGUAGE. Rows 2 and 3, in this staggered schema, designate MYTH. This is why Barthes refers to Myth as "metalanguage, because it is a second language, in which one speaks about the first" (p.115). Before we move on to an illustration of "myth" in action, it might be useful to walk this path from signifier to second-order signifier.