If we don't use return in chained promises, what will happening?

hi. in this exercise, and in the chained sample code :

.then((firstResolveVal) => {
  return secondPromiseFunction(firstResolveVal);
})
.then((secondResolveVal) => {
  console.log(secondResolveVal);
});

what will happens if we don’t return the promise.
i mean what the .then pass to the next .then ?