In the early days of the Internet people added metadata (data about data) to web pages to tell automated search-engine robots what was there.
Invisible to humans, this was a kind of table of contents for these automated web crawlers, who in turn report to the search engines. A whole science of search-engine optimization evolved to get search engines to notice content and rank it highly.
Today the robots have grown smarter. They read the web very much the way that we do. Metadata is all-but obsolete. Instead, to tell search engines about your web page, the best strategies involve clear organization and descriptive headings.