notr.tv is all about annotating videos.
Annotations are text (and eventually graphic) overlays that are played over the top of YouTube videos. Annotations also allow parts of a video to be skipped during playback.
What's it for?
Annotations can be used to add subtitles and comments to other people's videos, or to just cut out the boring bits.
One popular use is to add foreign-language subtitles to videos so that more people can enjoy them. After all, most internet users don't speak English as their first language.
Another fun application is to add fake subtitles to videos.
Other possible uses include making your own karaoke videos, creating subtitles for the deaf and semi-literate, and critiquing videos.
Essentially notr.tv is a way to personalize and share videos.