Primary objectives of the meeting are to identify three specific things:
Things the team needs to keep doing: best practices
Things the team needs to begin doing: process improvements
Things the team needs to stop doing: process problems and bottlenecks
These areas are discussed and a list of Agreed Actionable Improvements is created.
Reference:
http://blog.scrumstudy.com/all-about-retrospect-sprint-meeting/

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:
backlog/

Explanation
Affinityestimation is a technique used to quickly estimate a large number of User Stories. Using sticky notes or index cards or tape, the team places User Storied on a wall or other surface, in order from small to large. For this, each team member begins with a subset or User Stories from the overall Prioritized Product Backlog to place by relative size. This initial placement is done in silence. Once everyone has placed their User Stories on the wall, the team reviews all the placements and may move User Stories around as appropriate. The second part of the exercise involves discussion. Finally, the Product Owner will indicate some sizing categories on the wall. These categories can be small, medium or large, or they may be numbered using story point values toindicate relative size. The team will then move User Stories into these categories as the final step in the process. Some key benefits of this approach are that the process is very transparent, visible to everyone, and is easy to conduct.