[1] Its name refers to this two-level structure inherent to sign systems, many of which are composed of these two kinds of elements: 1) distinctive but meaningless and 2) significant or meaningful.
Double articulation[2] refers to the twofold structure of the stream of speech, which can be primarily divided into meaningful signs (like words or morphemes), and then secondarily into distinctive elements (like sounds or phonemes).
These sounds, called phonemes, represent the secondary and lowest level of articulation in the hierarchy of the organization of speech.
Higher, primary, levels of organization (including morphology, syntax, and semantics) govern the combination of these individually meaningless phonemes into meaningful elements.
The French concept of double articulation was first introduced by André Martinet in 1949, and elaborated in his Éléments de linguistique générale (1960).