a broad marketing term often describing different things, but in general links deemed to be harmful for search engine rankings in Google and other search engines at varying grades.
In short: Spam links are bad links.
Somehow, you acquired backlinks from sources or sites that use spammy links that point back to your site.
There are several reasons why spam links occur:
Unnatural linking has happened. In this case, you need to disavow links pointing to your site by uploading a file containing these url’s of these backlinks to Google. Some links are spammy and spammy links are automatically redirected to your site. Next you need to reach out to these source websites and either get them removed or ask them to no-follow the links.
Your site has been tagged as harmful or vulnerable to an exploit by Google’s quality guidelines. Make sure to update your CMS’s software, plugins, and themes to the latest versions.
It’s possible that you are part of a hacked links network, and even have such spammy links going out of your site. So make sure you remove malware and other online threats yourself or hire a reputable third party to scan your webpage for malicious code and links.
Your link profile is not natural or has AI style or automated content. Check to see if your links appear to be written by software rather than real people. Google calls them link schemes, so to rectify this you should update your content and your backlink profile with links that are not (or don’t appear) to be part of a link scheme.
Your links or website are the target of a negative SEO attack. People are out there to do harm to your business, if your link profile is unnatural back links coming from bad sites in instances of links pointing to nothing, false and (sometimes just plain weird) directory listings, malicious and/or impersonating domains, suspicious off-topic tags and links to scam you.
To resist negative links, practice link-catching and copy your link profile in Google Search Console on a regular basis to filter out links that aren’t doing any good.
If your link profile is damaged by a bad SEO attack, use all methods like the disavow tool to rectify. Only as last resort consider changing your URL and starting an all new backlink development process.
A special category of spam links are scraper spam links.
These are links automatically generated by blackhat software on spammy software with the purpose of attracting (little) organic traffic. The intention is often only to get a few percent of their scraper site to rank.