What is an Index?
An index is a database containing the webpages and other content a search engine wants to display in search results.
Search engines do not search the web in real-time. Instead, they regularly crawl the web to find new and updated content. When they identify a piece of content they may want to include in search results, they add it to their index.
When a person enters a search query into the search engine, the search engine retrieves the relevant results from its index, and displays it on the search results page.
The process of adding content to the index is called indexing. While indexing takes you a step closer to appearing in search results, there is no guarantee that the indexed content will appear in search results.