An index is another name for the database used by a search engine. It contains information on all the websites the search engine was able to find. If a site is not in a search engine's index, users will not be able to find it using that search engine. Search engines regularly update their indexes.
Once web pages are processed, they are added to search engines index if they are determined to be quality content. Indexed Pages Tool helps you count your website pages indexed by the search engines.