In practice
A crawler starts from known URLs, reads the links it finds, and queues what it has not seen. Anything it cannot reach through a link or a sitemap effectively does not exist.
Check what is actually being crawled in server logs rather than assuming: bots often spend their time on parameters, filters and pagination nobody wants indexed.
Why it matters
Crawling comes before everything else. A page that is never requested cannot rank, no matter how good it is.
Crawl problems are usually architectural, not content problems — which is why they get missed in content-led audits.
Related terms