More exactly it returns the left side if it is false-ish.
It is similar to the following:

return left and right
---------------------
if not left:
    return left
else:
    return right

It is called short circuit evaluation in Python and it is also available with or operator.

return left or right
---------------------
if left:
    return left
else:
    return right

Just keep in mind the following:

3 == 3 == 3
>>> True

# Not equivalent to
(3 == 3) == 3
>>> False

# Equivalent to
3 == 3 and 3 == 3
>>> True

or a list with only 1 element, return 0.

Please read the description again. It's pretty clear, isn't it?

That's because a string is, under the hood, an array of characters. When put into a set, it's basically how it's treated like.