Right now, the feature is still a work in progress, but it already looks pretty amazing. It would allow any app that plays audio/video through the MediaPlayer on your device to send that media to your Chromecast. It would work with local content on your device storage (including SD cards), or streamed media like podcasts.
Koush demoed the functionality using the TWiT.TV app. He didn't say when the functionality would be rolled into CyanogenMod, but it seems like the feature is working fairly well already, so it may not be too long.