Black Hat SEO is the use of various tricks to increase your website ranking in search results. Such tricks may include spamming other websites with links, putting an enormous number of keywords in the keyword tags, using invisible text on web pages, using a fake page that is not seen by the visitor but for search engine spider bots only and more.
As a whole, these are techniques that are used to get higher search rankings in an unethical manner.
Such techniques may have been useful in the past. Nowadays, you will be severely penalized by search engines if you try to use such tricks to improve your website search results ranking. By any means, it is not advisable to employ Black Hat SEO techniques as this will not lead to improving your website ranking. On the contrary – it will eventually result in lowering its ranking.