The Sprint Retrospective is mainly for the Team, Product Owner included.
Differently from a Sprint Review, the Sprint Retrospective focuses not on the Project, but on the Team and its processes (how they did what they did and how they worked as a team).
There are two things every member of the Team should always answer about the Sprint:
- What they think went well
- What they think needs improvement
As a way to wrap this event, the top 2 answers for the questions above should be highlighted and added as Action Items (things to continue doing or improving for the following Sprints).