It doesn't even necessarily ensure that. It ensures that you document your process(es) and, ostensibly, that you follow the processes. It doesn't nothing to ensure that the processes achieve any particular result.
I like the definition that Crosby uses. Quality is meeting the requirements. This both ensures that you have requirements defined for your product and that you meet them, all of them, all of the time.
It also gets away from the "better" defition of quality that is nearly useless. Things like saying that a Cadillac is higher quality than a Chevrolet. A Cadillac certainly has more features than a Chevrolet, but it may or may not be of higher quality.
Matt