Retrospective is my favourite part of the Scrum and most important one.
In biggest shortcut, retrospective is a event to do health check of your team and process. It allows you praise people for hard work and solve problems about things that didn't go perfectly well. Anyway , I am sure you know these things already (if not , check Scrum Guides ) .
This article helps you investigate reasons why people do not enjoy retrospective anymore and how to make changes to make retrospective valuable and enjoyable again.