When you have a YouTube-account and upload videos on there, you can "monetize" your video, which means that ads are run when someone plays your video and you th

When you have a YouTube-account and upload videos on there, you can "monetize" your video, which means that ads are run when someone plays your video and you then get some of the money that these ads make. De-monetizing a video means that Google doesn't give you that money anymore. This can either be as described in the article caused by your video containing inappropriate content for which Google assumes that advertisers don't want to advertise on it (in this case Google also stops playing ads on that video) or because of copyright infringements.