Black Hat SEO is the "bad SEO" or "unethical SEO".
Black Hat SEO involves using various tricks to cheat search engines and get the webpages ranked higher by using bad practices.
One of the tricks used by black hat SEO practioners involve including some hidden text in the web pages which search engines can see but not human readers. That way, human readers will see some text but search engines will see some other tricky text with lot of keywords meant to cheat them.
Another common black hat SEO technique is keyword spamming, which involve repeating the same keyword several times in the text to give inappropriate weightage for such keywords and get listed in the top of the search results.
Black hat SEO techniques may bring some quick traffic to the web pages for a short period. However, today's search engines are very smart and they would eventually detect such tricks and black hat SEO approaches. Once it is detected, search engines will penalise the site and they may be excluded from search results.
Black Hat SEO techniques can cause serious negative impact on the search engine ranking. Avoid Black Hat SEO but go for White Hat SEO.