In one sentence
A zero-click search is a search that ends without the user visiting any result, because the answer was delivered on the results page itself. AI-generated answers have sharply increased the share of searches that end this way.
Zero-click is why traffic is becoming an unreliable proxy for visibility. A brand can be described favourably in thousands of answers a month and see none of it in its analytics, because influence now happens before the click and often instead of it.
The practical response is to measure the answer itself: mentions, rank inside the answer, citations and share of voice, and treat sessions as one outcome among several rather than the scoreboard.