New Viewers are viewers who have never watched something on your channel before.

This does not mean "unsubscribed views". This means "people who have never watched a video on your channel before". How you view this stat depends on the video - if the video is just trying to engage your "existing audience", this stat isn't important. For "How to" and tutorial focused channels, this will often be higher than "Returning Viewers", as people might only need to watch one of your videos.

This isn't really a stat to optimize for, it's more informational. But, if you notice "New Viewers" is consistently low, it might be a good idea to try to make a video that appeals to a wider audience. This depends on your channel and your type of content, though. It's helpful to realize when your videos are really only reaching your existing audience.

As a big channel, you're probably more concerned about getting new viewers, so figuring out your ratio of new viewers to returning viewers can be helpful. But of course, if you're going viral, you're probably getting a lot of new viewers.