Ubuntu Restricted Extras is a collection of software that includes support for common audio/video codecs, Flash, DVD playback, Microsoft fonts, and Java.
Ubuntu Restricted Extras is a convenient bundle of software packages that are commonly needed to add extra functionality for daily use, multimedia, and content creation on Ubuntu Linux.
It includes support for popular audio/video codecs like MP3, AAC, H.264, as well as DVD playback libraries, Adobe Flash plugin, Microsoft fonts, Java runtime environment, and more. By installing this bundle, Ubuntu gains the ability to play most audio and video files out-of-the-box, including MP3 music, DVD movies, YouTube/online videos, etc.
The packages are termed "restricted" because they may have patent, copyright or other legal restrictions in some countries. However, they are fully integrated in Ubuntu and do not require any third-party repositories. The bundle aims to provide a smooth multimedia experience after a fresh Ubuntu installation.
In summary, Ubuntu Restricted Extras is a convenient addon that saves users the trouble of hunting down codecs, plugins, and runtimes to enable full media capabilities on their Ubuntu desktop or laptop.