Estimated number of unique people that watched your video

Basically, some people watch the view multiple times, or watch it in chunks. This results in more than 1 view per person. This stat is telling you how many actual humans watched your video.

Unique Viewers * Average Views Per Viewer ~= Total Views. The math isn't exact, since youtube rounds AVPV up, but this is good to get a better understanding of how people watch your video.