Indexing is Google reading a page and storing it in its index, the database it draws results from. A page that is not indexed cannot appear in Google at all.
Indexing is the second of Google's three stages: after it finds a page, Google reads it and stores what it is about in its index, the database it draws search results from. A page that is not indexed cannot appear in Google at all.
Google does not guarantee it will index a page, even one that follows its guidelines. It leaves out pages it sees as duplicates or as not useful enough, and groups near-identical pages so that one represents them (the canonical).
To check a page, use Search Console's URL Inspection tool, covered in how to check if a page is on Google. The three stages are explained in how Google finds and ranks a page.