Black Hat SEO refers to questionable SEO techniques. SEO Black Hats usually try to optimize the page off page and build a lot of backlinks on external pages. However, these pages then usually only include empty pages that are linked to the respective backlinks. Other practices are keyword stuffing, cloaking or even hidden texts. These were hidden using CSS or the text was written in white color on a white background.
When search engines detect these measures, ranking penalties can take place.
In white hat SEO, one adheres to the guidelines of search engine optimizations.