Application Root

This term refers to your application's root folder.

Your application root will have always have a subfolder called WEB-INF.

Consider the following folder structure, which contains 3 applications.

webapps/
    MyApp/           (this is the application root for the MyApp application)
        index.jsp
        WEB-INF/
            web.xml
    jsp-examples/    (this is the application root for the jsp-examples application)
        index.jsp
        WEB-INF/
            web.xml
    ROOT/            (this is the application root for the ROOT application)
        index.jsp
        info.jsp
        images/
            logo.jpg
            photo1.gif
        WEB-INF/
            web.xml

Note that the Application Root is different to the ROOT Application, which is the main application for your site. The ROOT Application, like all applications, has an Application Root as well. In this case <webapps>/ROOT/.