In the terms of PhpStorm, content is a collection of files with which you are currently working, possibly organized in a hierarchy of subfolders. The folder which is the highest in this hierarchy is called content root folder or content root for short. A project has at least one content root folder, by default it is the project folder itself.
Having several content roots enables you to work with files from several directories that do not have a common immediate parent. This is helpful when you use some static contents, for example, icons. You can just save them all in a folder and then specify this folder as an extra content root in several projects.
By default, all the files in a content root folder are treated as sources and are involved in indexing, searching, parsing, code completion, etc. To change this status, folders within a content root can be assigned to the following categories: