In simplest terms, the indicia is the USPS postage mark that takes the place of a postage stamp in EDDM.
It's wise to apply for an indicia when you way to pay for postage when sending bulk mail. This means you don’t have to run the mail through a meter, or buy stamps and affix them to the mail pieces. Anyone can apply for a permit.
The USPS has an application online you can download to apply. There is an annual fee you must pay to use the indicia.