Black Hat SEO is technique in order to increase your website ranking in search results. Such techniques may include spamming other websites with links, adding large amount of keywords in the keyword tags, using invisible text on web pages, using a fake page that is not seen by the visitor (hidden iframe) but for search engine spider bots only and many more. In short, these are techniques that are used to get higher search rankings in an unethical manner.
Previously, such techniques might work to boost your ranking but nowadays your website will be penalized by search engines if you try to use such techniqus. If you try Black Hat SEO techniques, there is a chance that your website gets banned by search engines.