
Codecs are software that allow different types of video/audio files to be played. Some codecs are proprietary — meaning that a license is required to use them. Some codecs are available under free licenses like the GPL.
Kubuntu does not come pre-installed with codecs to play proprietary formats like mp3, rm (Real Media™), and others. The necessary codecs can be installed from the repositories.
For more information on which formats are supported out of the box by Kubuntu and how you can install additional codecs for 'Restricted Formats', please read the Restricted Formats documentation on the community help website.