What is the purpose of indexing?

Indexing is updating the cached webpages in search engine database. Indexed webpages are now ready for search engine rankings.

In layman's terms, indexing is the process of adding web pages into Google search. Depending upon which meta tag you used (index or NO-index), Google will crawl and index your pages.

 Indexes provide a way to find data records instantly. Instead of piling through a bunch of data looking for a record that you need, you perform a search using an index and the results are usually returned instantly.

Indexing generally refers to getting your web pages (or other various forms of content from PDFs to videos) indexed in Google & other search engines. A search engine will crawl your website in search of content to add to its index.