Indexing is the process of adding your webpage details to Google's database, then only your site will appear on search result for related queries. Indexing webpages is done by crawling bot (which is an algorithm, designed to crawl through web pages which are public). By using this tool, you can find number of pages that have been indexed by Google. Robots.txt and sitmap files having greater influence on the number of pages being indexed by Google. Submitting your website to webmaster tools will also improve number of index.