Social Listening is mainly used in quantitative research to supplement and even replace conventional tracking. The term refers to the analysis of comments and images that people put online and into the public domain, e.g. on Twitter or Facebook or on specialist blogs or forums.
There are various platforms that search these social media and harvest comments, and images from them. For example Radian6, Netbase, Synthesio, Brandwatch etc ( the list if ever-growing). All platforms allow a user to conduct basic analysis looking at trends in mentions of keywords, creating word clouds and analysing 'sentiment'. More sophisticated platforms provide a better ability to filter and explore the data and some are specifically designed to help inform a client's social media marketing strategy.
There is expected to be a strong future role for the analysis of these forms of data by qualitative researchers. The bulk of comments you will see will seem superficial, trivial or random. But they do provide a window on the informal conversation around your topic across the web, which can usefully supplement data from other methods.