The metaphor comes from a castle: the wider the moat, the safer the value inside. Unlike a temporary lead from a clever campaign, a moat is built into the structure of the business, so advantage persists even as competitors try to imitate it. Common sources include network effects that grow with each user, switching costs that lock in customers, and proprietary assets rivals cannot buy.
Moats are not permanent. Technology shifts and new entrants can erode them, so leaders continually reinforce their sources rather than assuming today's protection lasts.