Audience targeting is not really an agreed-upon concept or definition in digital advertising context.
Nevertheless, audience targeting is sometimes used to refer to the practice of targeting users according to first or third party data related to these users. Audience targeting data is often opposed to contextual or brand targeting where it is globally the advertising vehicule or website which is choosen.
In fact, audience targeting and brand targeting are often combined. I can target only 20-25 years men on a given website.
In digital advertising we have seen a shift from content or contextual targeting towards audience targeting due to the emergence of ad exchanges and RTB. With RTB and ad exchange, audience is most often bought impression by impression and based upon users related data.