It means that the video is no longer available. This could be for a number of reasons:
1. The uploader deleted it or put it on private.
2. If it violated YouTube's guidelines. Violent, pornographic, hate speech, or dangerous videos are not allowed on YouTube, and will be removed from the website if reported.
3. Copyright. If copyrighted material was used in the video (music, clips from movies or tv shows, and any other copyrighted material), then it can be taken down by the copyright holder. Most of the time videos with copyrighted material aren't taken down, but are demonetized (meaning the uploader can't make money from the video), but sometimes the copyright holder will take the video down.