Python unpacking operator (*)

This is Iterable Unpacking. You may have seen it in other places to assign values to multiple variables from a single expression

a, b, c = [1, 2, 3]

This syntax includes a * to indicate that this variable should be a list containing the elements from the iterable that weren't explicitly assigned to another variable.

a, *b, c = [1, 2, 3, 4, 5]
print(b)
# [2, 3, 4]

So, what's going on in your example? There's only a single variable name being assigned to, so it's going to take all the items not assigned to another variable, which in this case is all of them. If you try just

*s='abcde'

you'll get

SyntaxError: starred assignment target must be in a list or tuple

Which is why that comma is there, as a trailing comma is how you indicate a single-value tuple.

The trailing comma is required only to create a single tuple (a.k.a. a singleton); it is optional in all other cases. A single expression without a trailing comma doesn’t create a tuple, but rather yields the value of that expression.