Quality control refers to the execution of the planned quality activities by the Scrum Team in the process of creating deliverables that are potentially shippable. It also includes learning from each set of completed activities in order to achieve continuous improvement. Within the cross-functional team, it is important to have the skills necessary to perform quality control activities. During the Sprint Retrospect Meeting, team members discuss lessons learned. These lessons act as inputs into continuous improvement and contribute to the improvement of ongoing quality control.
Quality is required not only in products, but also in processes. Quality assurance refers to the evaluation of processes and standards that govern quality management in a project to ensure that they continue to be relevant. Quality assurance activities are carried out as part of the work. In fact, quality assurance is a significant factor of the definition of Done. The deliverable isn't complete if appropriate quality assurance has not been conducted. Often, quality assurance is demonstrated during the Sprint Review Meeting.
Reference:
http://blog.scrumstudy.com/quality-control-and-quality-assurance-in-scrum/

Explanation
User Story is Done according to the agreed upon Acceptance CriteriA. Thus, the role of Acceptance Criteria in maintaining quality is critical and needs to be clearly understood by the team. It is the responsibility of the Scrum Master to ensure that the Acceptance Criteria for a User Story are not changed by the Product Owner in the middle of a Sprint. Partially completed User Stories are rejected as not Done and moved back into the Prioritized Product Backlog.

Reference:
https://www.mountaingoatsoftware.com/agile/scrum/meetings/daily-scrum