The opposite of White Hat SEO, these are tactics used to rank a website by violating search engine guidelines. For example, a blogger might stuff irrelevant keywords in a blog post.
Black Hat SEO prioritises rankings over user experience & has a negative impact on overall website quality. It will eventually result in lower search rankings or get your website banned altogether.
You can always use ethical SEO tactics to improve search rankings. Just remember that websites must mostly be designed for humans, not search engine spiders.