"Key metadata" refers to important information about something. It is the crucial or essential data that provides valuable context or insights. Full definition
It archives pages exactly as they appear online, stores key metadata, and creates a PDF output file. (lawsitesblog.com)
Early systems went as far as tracking key metadata for future analysis; but over time it was discovered that accuracy took back seat to expediency. (artificial-intelligence.cioreview.com)
[Update: Amazon has since made the subtitle field a unique field so technically it is now 8 key metadata fields for SEO.] (authorimprints.com)