Think about planning a trip. Take a map and mark out a path to a destination. A map will provide useful information about routes and connecting points. A map should also be accurate and provide a realistic expectation for the actual travel. After planning a trip and departing, what happens when a road is closed, or worse, a road doesn’t even exist? Routes have to change, plans have to change, and frustration occurs.

Before any SEO work can be fully consumed by search engines, a site has to be crawled by those engines. Web crawlers are automatic programs that browse the web automatically and methodically. They provide feedback to the search engine regarding the potential path (and pitfalls) a site visitor may see. If a crawler encounters any problems reaching pages on a site, there’s impact on SEO. A search engine can’t index a page that it is unable to reach.

Crawl errors can include pages not found, URLs not followed, URLs timed out, and HTTP errors. Any of these errors can prevent a web crawler from navigating the site properly. It is critical to constantly monitor crawl errors (Google Webmaster Tools provides this information for free) to ensure that any pages that should be crawled are accessible.

When a site is crawled successfully, the actual site matches the site map, much like the roads matching a roadmap. SEO efforts can be lost quickly if a search engine can’t get to the optimized content without encountering errors.