It's "Who's She, the cat's mother?" said when someone refers to someone as "She", intending to be dismissive, rather than using a name when the name is actually known. The phrase is intended as retort to the aforemention dismissiveness, ie "she" would be used when the name isn't known, as in the cat's mother would be a "she". Although it is also often used as a friendly correction to an accidental usage.
yeah, seems to be mostly used as an excuse to tell you off, when someone has nothing better to do.
you do not say the persons name every time you refer to 'her' in a sentence, yet some will pull you up on it - usually when they have lost the argument and wish to quickly change the subject....lol