Vygotsky's cognitive theory of development stems from social development theory. The latter proposes that behaviorism and interaction are the pivotal agents that develop cognition. In other words, the dynamics of communication enhance the development of mental processes.
The way in which this happens, according to Vygosky, is through the process that he calls "making meaning". This means that, as we listen and speak, the imagery produced by our brain, the sound to meaning connections that we make, and the symbol to sound correlations that form instantly as we speak develop our mental processes.
In summary, Vygotsky is a major proponent of the role of language in developmental processes. Interaction is key to mental enhancement and communication develops cognition due to the neurological connections and imagery that our brains produce upon being triggered by language in the form of meaning, sound, symbols, and imagery.