Crawl depth refers to how far down into a website’s page hierarchy a search engine spider crawls.
A website’s homepage is at the top of the hierarchy. Pages linked directly from the homepage are level 1 whereas pages linked from level 1 pages are level 2, and so on. Intuitively, the closer a page is to the homepage, the more significant it is, and therefore the importance of crawling it increases. Large websites with many pages and levels need a strong domain authority for the crawl depth to include the entire site. Usually, the crawl depth of initial and incremental crawls will not be the same.