On July 9, 2010, YouTube introduced the Original quality option to YouTube videos.
If a video on YouTube has the original quality option on it, it means that it has been uploaded to YouTube in a higher resolution then 1080p (1920×1080).
A couple of example resolutions are 2K (2048×1152), and 4K (4096×2304). When a video is uploaded with a higher resolution
than 1920×1080, it will get the quality options (240p, 360p, 480p, 720pHD 1080pHD, as well as original). See image below of example;
Because 4K represents the highest quality of video available, there are a few limitations that you should be aware of. First off, video cameras that shoot in 4K or 2K aren’t cheap, and projectors that show videos in 4K or 2K are typically the size of a small refrigerator. Watching these videos on YouTube will require super-fast broadband.
To view some 4K videos on YouTube, have a look at this playlist.