range-over-functions is the experimental new feature where a function can generate a sequence by executing a bit at a time, i.e.

  s := []string{"hello", "world"}
  for i, x := range slices.Backward(s) {
    fmt.Println(i, x)
  }

  func Backward[E any](s []E) func(func(int, E) bool) {
    return func(yield func(int, E) bool) {
        for i := len(s)-1; i >= 0; i-- {
            if !yield(i, s[i]) {
                return
            }
        }
    }
  }

I don't think anyone expects this to work in Go as it is today, it's just a new feature that will make the language more complex, but it will make certain kinds of programs simpler to write.

I should also note that the official name is "range-over-function iterators", I called it by a wrong name earlier.