They are supported by the MacOS and iTunes can convert to an iOS device friendly format.
Files supported by MacOS
From the iPad 2 user guide:
A Song, Video, or Other Item Won’t Play
The song, video, audiobook, or podcast may be encoded in a format that iPad doesn’t support. For information about the audio and video file formats iPad supports, go to www.apple.com/ipad/specs.
If a song or video in your iTunes library isn’t supported by iPad, you may be able to convert it to a format iPad supports. For example, you can use iTunes for Windows to convert nonprotected WMA files to a format iPad supports. For more information, open iTunes and choose Help > iTunes Help.
Message was edited by: J D Knight III, cleaned up latter part