Pierre Bourdieu's concept of linguistic market and linguistic capital is the application of his important concept of habitus to the realm of linguistic and more specifically discourse analysis. In a sense, the idea of the linguistic market is the consideration of social structures when talking about what Noam Chomsky defines as lingual competence. While competence for Chomsky is universal, in Bourdieu's sense it very much depends on your background, social status etc. lingual competence, for Bourdieu, in the context of the linguistic market, is the ability to speak to the point, in a manner that fits the circumstances and objectives within those circumstances. It's an ability to use the right words, right grammar, register, tone, body language and so forth in a manner that is favorable by the social structure of the linguistic market.
The linguistic market, as the name implies, in build on economic relation within which certain lingual capabilities have a higher currency than others, what Bourdieu refers to as linguistic capital. The linguistic market, according to Bourdieu, is both substantial in being a certain social situation and an abstraction in the forms of rules that regulate the value of lingual utterances and the spread, accumulation and reproduction of linguistic capital.
To understand Bourdieu's idea of the linguistic market and linguistic capital, think about how Oxford English gives a speaker, in formal situation, much more voice and credibility than someone who speaks, for example, Black English. This is determined by the formation of the linguistic market which gives certain ways of communication a more favorable currency in comparison with others. According to Bourdieu, the linguistic market, much like other markets, in never a free market, for power relations within it predetermine the standards according to which linguistic capital is allocated, thus preserving the rule of the elite (which always speaks the most prestigious language).
See summaries of Bourdieu's work:Pierre Bourdieu – The Historical Genesis of the Pure Aesthetic - summary and review