Returning Viewers are viewers who have watched something on your channel before.

This does not mean "subscribed views". This means "people who have 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 is important. In a way, this can be a more helpful number than your subscriber count. This tells you how many people watching this video are likely interested in your content - likely to watch another video (if your topics are consistent).

This isn't really a stat to optimize for, it's more informational. But, if you notice "Returning Viewers" is consistently low, you might not be building a loyal audience.

If you want to start a business off of your YouTube channel, it can help to have a loyal audience. If you're getting a lot of returning viewers, it's a good sign that you're building that loyal audience.