Indexing is a fundamental concept used to enhance the efficiency of information retrieval, both in traditional books and modern search engines. It involves creating a mapping between words or phrases and their locations within a text, similar to the alphabetical index found at the back of a cookbook, allowing users to quickly find specific content without scanning the entire document. This method is crucial for search engines, which use specialized data structures to store these mappings and improve search speed. By keeping the index in memory, the process becomes even faster, akin to memorizing the index of a stack of textbooks, thus significantly reducing the time needed to locate information in large datasets.