Black Hat SEO usually means using techniques of dubious integrity to get results. Most commonly associated with getting search engine rankings by tricking the system but black hat can also apply to almost any dishonest or non-transparent practice. These include getting fake Facebook fans, fake likes or comments and ratings as well as technical tricks.
With recent advances in search engine algorithms, black hat SEO has become both difficult & risky and many of the techniques commonly used (some of which are still sold to naive webmasters) will actively undermine your search position.