HI there,
Thanks for reaching out to Adobe Community.
A visitor is included in the "Visitor: New" segment if it is the visitor's first time visiting the site, the first time since clearing cookies, the first time since the mbox.js file was installed, or if it has been two weeks since the last visit.
Please go through the below doc to know more.
https://marketing.adobe.com/resources/help/en_US/target/target/c_target_visitor.html
Thanks!