This is a built-in template used for filling the body of an implemented method each time it is generated by the program, e.g. when using the Implement Abstract Class/Implement Abstract Method intention actions or calling the Code | Implement Method... function (Ctrl+I).
The template is editable. Along with Java expressions and comments, you can also use the predefined variables that will be then expanded into the corresponding values.
Predefined variables will take the following values:
${RETURN_TYPE}   a return type of a created method
${DEFAULT_RETURN_VALUE}   a value returned by the method by default
${METHOD_NAME}   name of the method that is implemented
${CLASS_NAME}   qualified name of the class where method is implemented