A launch check is a short list of things you look at just before and just after a site goes live, to be sure nothing is stopping Google from finding and showing your pages. A site can look perfect in the browser and still carry a hidden instruction that keeps it out of search results.
Google's site move guide notes that some site owners block all crawling, or use noindex rules, while a site is in development. That is reasonable, but the risk is that the rules are still switched on at launch. This guide gives you an order to check them, using your browser and Google Search Console.