It's a function annotation; function arguments and the return value can be tagged with arbitrary Python expressions. Python itself ignores the annotation (other than saving it), but third-party tools can make use of them.
In this case, it is intended as type hint: programs like
mypy can analyze your code statically (that is, without running it, but only looking at the source code itself) to ensure that only str values are passed as arguments to splitComma.
A fuller annotation to also specify the return type of the function:
def splitComma(line: str) -> str:
...
(Note that originally, function annotations weren't assumed to have any specific semantics. This is still true, but the overwhelming assumption these days is that the annotations provide type hints.)