An indicia is an imprint on a mailpiece, that is instead of a stamp, indicating that postage has been paid. All pieces in a permit imprint mailing must be the same weight. The mailer must submit forms and a fee to the Post Office where the pieces will be mailed.
TIP: Many print vendors handle this as part of their mailing and fulfillment services.
For more info, see this USPS Quick Service Guide on permit imprints.