In a nutshell, with x = [1,2,3], when calling f(x), x receives 1 argument [1, 2, 3]. When you use the star operator f(*x), f receives three arguments, it is equivalent to the call f(1,2,3).
This is why, in Python's documentation, you will often see some_function(*args, **kwargs). Here the double star operator does the same, but for a dictionary: with d={"some_arg":2, "some_other_arg":3}, calling f(**d) is the same as f(some_arg=2, some_other_arg=3).
Now when you use zip, effectively you want to zip [1,2,3] with [4,5,6], so you want to pass 2 args to zip, therefore you need a star operator. Without it, you're passing only a single argument.
