What it checks
Eight things, in the order they matter. Whether the site answers over a secure connection. Whether your robots.txt shuts out the assistants that fetch pages to answer a question. Whether the home page tells them not to list it. Whether the words arrive with the page or only after scripts run. Whether the page has a title and a description. Whether it describes itself in structured data. Whether there is a sitemap. And whether there is a plain index for assistants.
It also notes three things for information only. Whether crawlers that collect text for training are shut out, whether the page offers a plain text copy, and whether the site describes anything an assistant could do for a person, not only read.
What the result means
Fix this means an assistant cannot read or list the site until it is changed. Could be better means the assistant can read the site but has less to go on, so it may describe you loosely or leave you out.
Passing every check does not make an assistant recommend you. It means nothing stands in the way. What it says about you then depends on what your pages say, how specific they are and who else writes about you.
What it does not do
It reads the home page and a few public files. It does not crawl the whole site, log in, fill in forms or test anything behind a password. It does not store the address you type or what it found.
It cannot tell you how often assistants mention you today. That needs the same buyer questions asked week after week, which I do for my own site and can set up for yours.