Toxic links are backlinks from low-quality, spammy or irrelevant sites that can hurt your site's ranking. Common examples: links from spam directories, hacked sites, private blog networks (PBNs), gambling or adult sites (with no relevance to your industry) and links with manipulative anchor text.
To identify toxic links, use tools like Ahrefs, SEMrush or Google Search Console. Look for: very low-authority sites linking to you, links from domains in languages unrelated to your business, sudden spikes of new links (which can indicate a negative SEO attack) and an excess of exact-match anchor text.
If you find toxic links, you have two options: try contacting the webmaster to request removal, or use the Disavow tool in Google Search Console to indicate that you do not endorse those links. Disavow should be used with caution and only for clearly harmful links.