How do Author keywords and Indexed keywords work?

From a Document details page, you can view Author keywords and Indexed keywords:

• Author keywords are chosen by the author to best reflect the content of the document.
• Indexed keywords are chosen by Scopus and are standardized to vocabularies derived from thesauri that Elsevier owns or licenses. Unlike Author keywords, Indexed keywords take into account synonyms, various spellings, and plurals.