How to use optional argument of random.shuffle in Python

From the doc of random.shuffle(x[, random]), it says:

The optional argument random is a 0-argument function returning a random float in [0.0, 1.0); by default, this is the function random()

Could someone please explain what 0-argument function means and give an example of random.shuffle() with the optional argument random? I searched but couldn't find any example for that case. Also, what did it mean by this is the function random()? Does this refer to the optional argument?

Thank you.