Crawl depth is the extent to which a search engine indexes pages within a website. Most sites contain multiple pages, which in turn can contain subpages. The pages and subpages grow deeper in a manner similar to the way folders and subfolders (or directories and subdirectories) grow deeper in computer storage.
In general, the further down in the Web site hierarchy a particular page appears, the smaller the chance that it will appear with a high rank in a search engine results page (SERP). A Web site's home page has a crawl depth of 0 by default. Pages in the same site that are linked directly (with one click) from within the home page have a crawl depth of 1; pages that are linked directly from within crawl-depth-1 pages have a crawl depth of 2, and so on.