In simple words, processing or rendering of the content of the web pages which search engine gets from crawling is called search engine indexing.
Indexing is next step after crawling which Google and other search engines take note of key signals — from keywords to website freshness — and we keep track of it all in the Search index.
According to Google
“The Google Search index contains hundreds of billions of web pages and is well over 100,000,000 gigabytes in size. It’s like the index in the back of a book — with an entry for every word seen on every web page we index. When we index a web page, we add it to the entries for all of the words it contains.”
Search engine Index process everything which is found on a webpage like keywords, images, headings, links etc. Google search Index compiles all the words it sees on a web page and their location on each page and stores it at a database of billions of pages.
The stored words of web pages interpreted by search engine algorithm to measure importance compared to pages similar to it. After this, search engines decide to rank a page for given keywords and how it should be found in their search result.
Search engine crawling is a process of finding a web page by following links and indexing means processing information (words, images etc) of the page to decide its ranking.
Crawling and indexing both are different things if a page is crawled then it doesn’t mean it also gets indexed. But, every indexed page had to be crawled.