Broken links are the external website to which your website is linked but now no more exist.
So now if anyone visits your site instead of that linked page website show a 404 not found error which is part of bad SEO practice.
To avoid such a link from your site using a Broken link checker you must be having an idea which links Status is 200,301 or 404.
For those which are having 404 status, you need to either remove that link or link with any other content. So it will make your job easier to fix the SEO issues.