In Scrum, quality is defined as the ability of the completed product or deliverables to meet the Acceptance Criteria and achieve the business value expected by the customer.
To ensure that a project meets quality requirements, Scrum adopts an approach of continuous improvement whereby the team learns from experience and stakeholder engagement to constantly keep the Prioritized Product Backlog updated with any changes in requirements. The Prioritized Product Backlog is simply never complete until the closure or termination of the project.
Any changes to the requirements reflect changes in the internal and external business environment and allow the team to continually work and adapt to achieve those requirements. Since Scrum requires work to be completed in increments during Sprints, this means that errors or defects get noticed earlier through repetitive quality testing, rather than when the final product or service is near completion. Moreover, important quality-related tasks (e.g., development, testing, and documentation) are completed as part of the same Sprint by the same team—this ensures that quality is inherent in any Done deliverable created as part of a Sprint.
Thus, continuous improvement with repetitive testing optimizes the probability of achieving the expected quality levels in a Scrum project. Constant discussions between the Scrum Core Team and stakeholders (including customers and users) with actual increments of the product being delivered at the end of every Sprint, ensures that the gap between customer expectations from the project and actual deliverables produced is constantly reduced.