A role based email address is one that usually has a generic prefix and can go to many people in a company at once. These would be addresses like info@, sales@, admin@, etc.
These types of email addresses have historically caused delivery problems with high bounces, the chance for abuse or even sending unwanted emails to some people on the list. We do our best to maintain high delivery rates so your recipients can receive your emails. Because of this, we automatically remove these role-based email addresses during the list upload process to keep your bounce rates low and your open rates high.
You can see when these addresses get pulled out during upload by looking under the Not Imported side where it says Nonmailable Address.