Indexing is a search engine storing a page in its database after crawling it. Only indexed pages can appear in search results.
It happens in steps: the engine discovers a URL through a link or a sitemap, crawls it, then decides whether it is worth keeping. A page can be crawled and still not indexed if it is thin, duplicated or blocked.
For a new site this takes days to weeks. Links from sites that are crawled often speed up discovery.
Why it matters for a launch
When a new site is "not on Google", it is almost always not indexed yet. A sitemap, a few backlinks and Search Console's URL inspection are how you fix that.
Read more: Why a new website isn't on Google yet