Search engine optimisation has acquired a reputation for mystery that the underlying mechanics do not deserve. A search engine has to reach your page, retrieve its content, understand what the page is about, decide whether to keep it, and then judge it against everything else it holds on the same subject.
Each of those steps can fail, and they fail for different reasons. Reaching can fail because of directives, architecture or errors. Understanding can fail because a page never states plainly what it is about. Judging can fail because five of your pages make the same claim and none of them is clearly the authority.
This module works through those steps in order and reports where the chain breaks, because a failure early on makes everything downstream irrelevant.