An index is another name for the database used by a search engine. Indexes contain the information on all the websites that Google (or any other search engine) was able to find. If a website is not in a search engine’s index, users will not be able to find it.
The index as the main reference point source data in Google’s index that ultimately determines the value of different search terms and keywords. Search engines apply their algorithms to the available data, and measure the frequency of different factors under different conditions, which factors are related to one another and so on. The index includes not just the URLs, but all content, including texts, images, videos and, in principle, everything within the HTML code of the URL.
The information garnered from this analysis flows back into Google’s algorithm to provide a new assessment of the index data, which attempts to understand which content best meets which user intent. The Google search results, or rankings, are then calculated on the basis of this content assessment.
Here is some indexing site list