Indexation is the process of a URL being picked up by a search engine crawler bot & being added to their index of search results, where they will be assessed for rankings.
Crawlers follow URLs around the web, indexing any content that is not of poor quality or that is not explicitly said to not be indexed by the site’s directives.
When content is indexed, it is placed in the ranking system, where it is assessed & then ranked to be featured in Search Engine Results Pages (SERPs).
Always be sure that your directives are blocking bots from indexing content that you do not want appearing in search results.
Conversely, ensure that all directives are not keeping content you want found in search from being indexed.