The purpose of indexes in a database is to provide an efficient way to find and access data. Books provide a good analogy here. If we have a book of short stories and we want to find one that is about animals, without an index we would need to flip through every page of the book and read it to see whether any animals are mentioned.
If the book had an index on the subject, that would be a much more efficient way to find the story we are looking for. An index in a book is typically sorted alphabetically, so we wouldn't even need to read the entire index, we could just go to the A section and find Animals and then stories about animals would be listed with their page numbers. If all we want is to know is whether the book contains animal stories, we're done with just a quick glance at the index. If we want to actually read the story, we can flip to...
