Known shippers are shippers who have been properly vetted with the TSA (Transportation Security Administration) by an air carrier or freight forwarder.
Once these shippers comply with the current security requirements and receive the proper approval, they are able to book rates on passenger aircrafts as well as on cargo planes.
The Known Shipper Program was implemented after the 9/11 attacks by the United States Department of Homeland Security and the TSA. It is heavily regulated by the TSA through the Known Shipper Management System.
For questions about becoming a known shipper, you can email the TSA at KSMSSUPPORT@tsa.dhs.gov with the subject: "Known Shipper Management."