New visitors on your site are the people who land on yoir website for the first time. Returning visitors are the people who come to your site second time, third time, etc. When you get returning visitors, it signify that people like your content. You can convert new visitors to returning visitors by building useful and interesting content. But the main question is what kind of visitors are good for your site in terms of revenue generation? Will you earn more if you have new visitors or if you have returning visitors? The revenue generation I am talking about is from ad impression and ad click.