In agile, we talk about the quality at all stages in contrast to the waterfall where attention to quality was being given more towards in the beginning PART of the SDLC rather than later on. We make sure that in Agile, we have certain CHECKPOINTS to make sure whatever goes out, is as the quality standard. And hence, we set the definition of done where we set the parameters on quality. 

This definition of done is made as per the agreement between the team and the stakeholders and is fixed for a sprint (at the minimum). The stories COMMITTED by the team can only be marked as complete once it meets the criteria defined in the definition of done. In the definition of done, the team can set unit testing, code review, coverage, etc. as the parameters, if the team is working on accessibility, they can add the criteria in terms of compliance. Hence, the quality is frozen at the initial level so that whatever REQUIREMENT is shipped, it should adhere to the set norms. In the same way, we can have a quality BACKLOG to be entering into the sprints with the help of definition of ready.