In short, Click Depth is the number of clicks from the homepage before the user arrives on another page.
Click Depth is very important from a search engine optimisation perspetive. If you have a large ecommerce site, and your product pages are 4-5 clicks from the homepage, from a search engines perspective, your product pages are not as important as they are lower down your sites architecture.
Here is what Google’s John Mueller said on the subject:
“What does matter for us … is how easy it is to actually find the content. So especially if your homepage is generally the strongest page on your website, and from the homepage it takes multiple clicks to actually get to one of these stores, then that makes it a lot harder for us to understand that these stores are actually pretty important.”
“On the other hand, if it’s one click from the home page to one of these stores then that tells us that these stores are probably pretty relevant, and that probably we should be giving them a little bit of weight in the search results as well. So it’s more a matter of how many links you have to click through to actually get to that content rather than what the URL structure itself looks like.”
Click Depth/Page Depth are interchangeable and essentially describe the same thing, but different nomenclature.