the.com/keynote

means The central theme or opening address that sets the tone for everything that follows, whether a conference, a speech, or a strategy.

from Straight out of music theory — the 'key note' (sometimes 'keynote') is the tonic, the single note that establishes a piece's key and to which all other notes relate. By the 1700s English speakers were already borrowing it figuratively for the central idea of a speech or document — the note everything else is tuned against. The 'keynote speech' or 'keynote address' extended that logic: the talk that sets the pitch for the whole event, even if, as the essence notes, it's the only part anyone remembers.