The extent to which search engines index your site content. A site with good internal linking will help crawlers jump around the website, understand the site’s structure, and find new pages. Crawlers will have difficulty navigating your site if they can’t follow internal links around. To improve crawl depth, you can use breadcrumb navigation, maintain your XML sitemap (i.e. give Google a directory of the URLs it should index), and make improvements to your site’s speed. If the crawler detects slow load times, it’ll stop following links around your site.