Modules have at least one content root folder, also referred to as a content root for short. All the files that make up your module must be within its content root (i.e., inside the content root folder).
A module can have more than one content root, however, in most of the cases one content root will suffice.
Folders within content roots may be assigned to the following categories:
For Java, the top of this folder is considered the root of a package hierarchy; .java files in it are considered part of the default package, whereas subfolders designate a standard Java package structure.
A source root folder can contain the files that are not source code files (.xml, image files, etc.). When building a module, such files, generally, are copied to the output folder as is.
Source folders in IntelliJ IDEA are shown blue .
Test source folders in IntelliJ IDEA are shown green .
Compilation results for sources and test sources, normally, are placed into different folders.
Excluded folders, when appropriate, are shown pink .