What is a single * in function or method? (Python)

Question

I already know about *(iterable unpacking) and **(dictionary unpacking) for unpacking and also pass as an parameter for a function or method.

But in many cases I see someone using * without any name like this example below:

def simple_func(arg1=None, *, data=None, ...):
    ...

What this meaning?

Thanks in advance!