a directory of content
A mapping of unstructured data (the content on your website) to structured data, so that the correct unstructured data can be quickly found when looking for it.
A "Rolodex", card catalogs in libraries, phone books — all the analogs we have are so old that they don't really work very well even as metaphor, any longer. Indexes are basically invisible to us, now, all digital, hidden away inside our magic screens.
Google provides several, often very highly relevant, possible answers to your question in a fraction of a second, from hundreds of thousands of other possible answers, by means of a truly mind-blowing index.