If a visitor comes to your website for the first time, or if he visits a page (or downloads a file) more than 30 minutes after his last page view, this will be recorded as a new visit. A Unique Visitor can make multiple Visits per day.
In the first visit, the visitor is a New Visitor. From the second visit (the same day or later), a Unique Visitor is a Returning Visitor.
You can change the 30 minutes value (session timeout) by overriding

[Tracker]
visit_standard_length = 1800

in your config/config.ini.php file.