Crawl errors are unsuccessful attempts by search engine spiders to crawl a website.
Google’s Search Console divides crawl errors into two categories: Site Errors and URL Errors. Site errors affect an entire website. They include problems with DNS, server connectivity and the robots.txt file. URL errors affect specific URLs. They include Soft 404 errors, 404 errors, Access Denied, and Not Followed. Significant amounts of crawl errors point to poor website health, and that can negatively affect the user experience as well as search engine rank, crawl frequency, and crawl depth.