When you have once a project in Unity, this structure of the whole project is displayed on the left side. When you download the Standard Assets package, you need to be careful how you import it. If you look at the project structure on the left, then you can also see the Readme folder, where the Readme file in pdf format is located and it explains how to properly import Standard Assets into your project and then do the import process so that everything works properly, when you start a project. In addition you also have the Documentation folder, which contains the Documentation file in pdf format, which describes a complete explanation of how the project is composed and how to implement the game with all the images in Unity.