Retrospective is a ceremony that happens at the end of each sprint. The retrospective has 3 parts:
The first part of the retrospective is the scrum master asking about each card that isn't finished to see if they can be moved over. Unfortunately, every team will have some lazy members that don't move cards over when they finish.
The second part of the retrospectve is demos. Any member who wants to show off the work they've done during that sprint can show it off to the rest of the team. Demos should be kept under 5 minutes. Time is money.
The last part is the most important part. The scrum master will create two lists. One list is "What went well" and the second is "What can be improved". Each member of the team should contribute to the list 1 thing that went well 1 thing that did not. When the lists are complete the scrum master can highlight any of the positives from the "what went well" list. Then the team can then vote on which of the "What can be improved" to discuss. It sometimes can be better to have 2 votes: first vote is what in the "what needs to be improved" list is the most important to the team and the second is what on the improved list should the group talk about. The reason for this is sometimes the thing bothering the group the most cannot be solved by the group (things such as layoffs). The scrum master could take something like that to upper management to let them know it's an issue.