What exactly is search engine indexing?

after google crawls a page it decides whether to store it in its index, which is basically the giant database of pages it can show. a page that isn't indexed can't rank for anything, however good it is. you can check any URL with the inspection tool in search console. "crawled - currently not indexed" means google saw it and chose not to keep it, which is usually a quality or duplication thing rather than a technical bug.