The signal read-only property of the NavigateEvent interface returns an AbortSignal, which will become aborted if the navigation is cancelled (e.g. by the user pressing the browser's "Stop" button, or another navigation starting and thus cancelling the ongoing one).

An AbortSignal object.

The general idea here is that the signal property can be passed to an associated fetch() operation so that if the navigation is cancelled, the fetch() operation can be safely aborted, avoiding wasting bandwidth on fetches that are no longer needed.

navigation.addEventListener("navigate", (event) => {
  event.intercept({
    async handler() {
      // ...

      await fetch(`/img/some-image.jpg`, { signal: event.signal });

      // ...
    },
  });
});

• Modern client-side routing: the Navigation API
• Navigation API explainer
• Domenic Denicola's Navigation API live demo