What does -> mean in Python function definitions?

I've recently noticed something interesting when looking at Python 3.3 grammar specification:

funcdef: 'def' NAME parameters ['->' test] ':' suite

The optional 'arrow' block was absent in Python 2 and I couldn't find any information regarding its meaning in Python 3. It turns out this is correct Python and it's accepted by the interpreter:

def f(x) -> 123:
    return x

I thought that this might be some kind of a precondition syntax, but:

• I cannot test x here, as it is still undefined,
• No matter what I put after the arrow (e.g. 2 < 1), it doesn't affect the function behavior.

Could anyone familiar with this syntax style explain it?