High quality means a low defect rate. Customers never lose confidence in the product. Brand reputation is maintained and held in high regard. Any bugs which are found have little if any impact to production. High quality and low defect rates are achieved through a combination of test driven development and for manual testing a focus on the most important and most likely areas which will cause trouble.
With test driven development developers write automated tests for their code prior to coding, then code to pass the tests. These tests are then stored for automatic execution against the code base. When coding changes are made and checked in the automated tests point out any issues. Since it is often the case that there must still be manual testing, Analysts focus on those tests which are critical to the system, creating these tests first and subsequent tests are developed as coding progresses.
With a focus on the most important, high value customer requirements, the features most used and those delivering the greatest return on investment are delivered bug free. This combination makes for a very happy customer base even early in the development effort.
Experience plays a key role in knowing where the majority of testing should be focused. Automated tests can be run any time. It is the longer non-automated testing which requires experiential focus and direction in order to deliver a high quality system. Key to high quality is having an experienced business process owner as well as a senior development team leading the effort. Software is costly to develop, but it cost much more when things to do not go well and development runs over scheduled delivery dates. Paying for experience up front in development is a good return on investment.