Message match is continuity from the promise in the ad to the experience after the click. The page does not have to repeat the ad word for word, but the visitor should recognize the same audience, problem, offer, next step, and tone. The plain test: the page should answer five questions in the first screen, who this is for, what problem it addresses, what the visitor gets, why the next step is credible, and what happens after they act. You can read that framing in full at Click Laboratory's guide to message match.
Keyword repetition is the shallow version. A page can contain the exact phrase from the ad and still break the promise, because the visitor's reason for clicking sits in the offer and the qualifier, not in a noun. Unbounce puts message match first among its landing page best practices and warns that an ad for retirement communities pointing at a page about luxury condos will drive visitors away, which is the same failure at a different scale. See Unbounce's landing page best practices.
One more reason this matters more than it looks: people scan, they do not read. Nielsen Norman Group found that 79 percent of test users always scanned a new page and only 16 percent read word by word, and the same scanning behavior held across eyetracking studies spanning 13 years and more than 500 participants (how users read on the web, how people read online). A mismatch buried in paragraph four is invisible. A mismatch in the headline or the first button is the whole page.



