Resource bundles contain locale-specific objects. When your program needs a locale-specific resource, a
String for example, your program can load it from the resource bundle that is appropriate for the current user's locale. In this way, you can write program code that is largely independent of the user's locale isolating most, if not all, of the locale-specific information in resource bundles...
More at java doc
Resource Bundle is nothing but Localisation and Internationalisation.
Whatever written in the UI, in which language where should it appear is taken care by Resource Bundle.
For ex. If you have to write Custom.Action file you need to extend the resourceBundle.