A keynote is "the leading note in a musical key," and that meaning has spread to include the leading theme or idea in almost anything. For example, a keynote speaker at a conference includes this central subject in her speech.
You're most likely to find the word keynote followed by "speech," "address," or "speaker." The central keynote theme is often presented right at the beginning of a conference or seminar. Another meaning of keynote is "the leading note in a musical key." When you play a scale on the piano, the first, lowest, note is the keynote. This musical meaning dates from the mid-1700s, and the phrase "keynote address" was first used in 1905.