What is a retrospective?
A retrospective is a way to take a look at how something went, such as a sprint, a release, or some other period of work. For example, you can make a habit of holding a retrospective meeting immediately after each sprint, where you discuss how the sprint went, and create a retrospective for the sprint.
A retrospective consists of columns where you add cards that represent observations, ideas, to-do items, etc. There are many ways to organize retrospectives, in terms of how many columns you use and the purpose of each column. Here's an example for a sprint:
In the first column, you add cards for the things that went well in the sprint. In the second column, you add cards for the things that didn't go so well. And in the third column, you add cards for the improvements that you want to make.
But choose whatever format that works well for you. And you might also want to change the format from time to time, to create some variation.