Usually, when you create a project you will also need to create various folders to keep everything organized within the Project window. However, when naming your folders, there are a few names that Unity will interpret as an instruction for the folder's contents to be treated in a certain way. For example, for Editor scripts to work you must place them in a folder named Editor.

The folders Assets, Editor, Editor Default Resources, Resources, Gizmos, Plugins, Standard Assets, StreamingAssets, and Hidden Assets, which are described in more detail later, are not automatically created within Unity. Therefore, each time that you create a new project, you will need to create them manually. Keep in mind the names will trigger Unity to treat them accordingly, so be sure to not use them if they're not going to do what you intend them to. Here is a screenshot of what the Unity Project window would look like if you have created all the special folders.