What is Search Engine indexing?

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Search Engine indexing is the method by which search engine copies the information of certain pages in its cache server. Search Engines have bots known as crawler. Crawlers crawl through the page created in a website, and saves them to its cache server. 

Why is it saved into cache server, what does that mean? 
 Well, the Internet is dynamic; it is changing all the time. It need to be saved in the temporary memory so as to update,if the information in site updates. 

Why do indexing necessary?
 It is neccessary to make the search result quick and efficient. If a server tries to search for a keyword in real-run-time, then it will takes hours to generate the result. When the data is stored in the server itself, then it takes parts of a second to generate the result to an user.