How crawl depth is measured
Start at the homepage, follow internal links, and count the steps to the page you care about. A service listed in the main menu sits one step away. A blog post reached from the menu, then a category, then the third page of results sits several steps away. That count is the crawl depth, and it is also called click depth.
The important thing is that this is measured by following links, not by reading addresses. A long URL with several folders in it can still be one click from the homepage, and a short address at the root can still be buried where nothing links to it. Confusing the two is the single most common misunderstanding of this term. To know the real figure you have to run a crawler such as Screaming Frog from the homepage and read the depth it reports.
Why crawl depth matters
Search engines follow links, so depth affects how easily a page is found and how often it is revisited. Pages close to the homepage are seen on almost every crawl. Pages far from it are reached less often, so changes to them take longer to register.
Depth also tracks how internal value flows. Links pass importance through a site, and each additional step shares that value among more pages, so a page at the end of a long chain inherits very little. Deep placement is, in effect, a statement that the page is not one of your priorities.
And it matters for people, who behave the same way. If a customer has to work through several listing pages to reach a service you want to sell, most of them will not arrive. The old rule of thumb that nothing important should be more than a few clicks from the homepage is crude, but it points at something real.
Where crawl depth goes wrong
Pagination is the usual culprit. A blog or catalogue where the only route to older items is page after page of results pushes everything old to the far end of the site, whatever it is worth.
Filters and facets create the same problem in a different shape, generating long chains of near-identical URLs that swallow crawling while the pages that matter sit behind them.
The opposite error is flattening everything. Linking every page from the footer to make the numbers look good produces a site with no structure, where nothing signals what belongs to what. Depth should reflect a real hierarchy, not be engineered away, and site architecture is the thing you are actually designing.
Finally, depth quietly changes when the menu changes. A page dropped from the navigation during a redesign can move from one step to many overnight, and nobody notices until traffic to it fades.
What to do about it
Crawl the site and list your commercial pages by depth. Anything you sell, anything that generates enquiries and anything seasonal should be close to the surface, reachable from the homepage or the main navigation.
For the rest, build routes rather than shortcuts. Category and topic hub pages, breadcrumbs, and links from one article to related articles all pull deep content upwards while keeping the structure meaningful. That is ordinary internal linking work, and it usually helps visitors more than it helps crawlers. Re-run the crawl after any redesign or menu change, because that is when depth shifts without anyone deciding it should.