These preview pages are shown to you before every stage of a project.
Here you can find a list of topics that you need to learn for successful completion of the project stage. Some of the topics you may already know, so you can skip them.
Under Show details, you will find a description of a stage and examples of how it should work (output/input-wise). It helps to see what sort of theoretical and practical knowledge you will acquire during this stage and put things in perspective.