What does ** (double star/asterisk) and * (star/asterisk) do for parameters?

What do *args and **kwargs mean in these function definitions?

def foo(x, y, *args):
    pass

def bar(x, y, **kwargs):
    pass

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_{See What do ** (double star/asterisk) and * (star/asterisk) mean in a function call? for the complementary question about arguments.}