It is a meta-package that installs:
Support for MP3 and unencrypted DVD playback Microsoft TrueType core fonts Flash plugin codecs for common audio and video files
The Medibuntu project has similar aims to the ubuntu-restricted-extras package. Contents
The ubuntu-restricted-extras is a metapackage and has the following dependencies:
flashplugin-installer gstreamer0.10-ffmpeg gstreamer0.10-fluendo-mp3 gstreamer0.10-pitfdll gstreamer0.10-plugins-bad gstreamer0.10-plugins-ugly gstreamer0.10-plugins-bad-multiverse gstreamer0.10-plugins-ugly-multiverse icedtea6-plugin libavcodec-extra-52 libmp4v2-0 ttf-mscorefonts-installer unrar
As of Ubuntu 10.10, several of these dependencies are included indirectly, through another metapackage called ubuntu-restricted-addons.