Google uses PageRank to determine a page's importance by counting the number and quality of links pointing to it. Pages deeper within a site have less internal PageRank because they are linked less often.
It refers to how many clicks you need to take to get to a specific page from the homepage using the shortest path. A page directly linked to the homepage has depth 2 (the homepage itself is always 1!).
An accessible page only takes one click to reach, so it performs better than a page that requires more than three clicks.
Because deep pages are difficult to find, crawlers won’t check them as often as pages at depth 2 making them less likely to rank.
A tree graph can help you visualize the overall structure of your website and identify problematic pages.