Indexing is a fundamental process in the work of search engines, which follows the scanning phase (Crawl). It is a systematic registration, classification and entry of all content collected by search engine robots into the main database (index). This index can be thought of as a giant, structured library that stores copies of billions of web pages, sorted by keywords and semantic context.
Conceptually, indexing is the act of including a web page in the potential set of search results. If a page is not indexed, it does not exist for the search engine, regardless of its quality, and will never appear in the SERP. The process involves analyzing the text, images and metadata of the page to determine its thematic relevance and place in the search structure.
Successful indexing is a critical condition for SEO. Technical optimization is aimed at removing any obstacles (such as noindex directives or robots.txt issues) that may prevent the search engine from processing and entering the content into its index.