Three important points in this definition
Explicit software requirements are the foundation from which quality is measured. Lack of conformance to requirements is lack of quality
Specific standards define a set of development criteria that guide the manner in which software is engineered. If the criteria are not followed, lack of quality will most surely result
There is a set of implicit requirements that often goes unmentioned (e.g., ease of use). If software conforms to its explicit requirements but fails to meet implicit requirements, software quality is suspect