A backlink is a link to your website from another website. It "links back" to your website. Backlinks from reputable websites are good for your website's rankings. Backlinks from "bad" websites are bad for your website's rankings. One should occasionally monitor the links to one's website and disavow any links from disreputable websites. Note that one Black Hat SEO technique is to flood a competitor's website with bad links from disreputable websites.