What does the TSA mean by known traveler number?

The "Known Traveler Number" is a number assigned by the Transportation Security Administration to passengers accepted into the TSA security pre-check program. Passengers should add their KTNs to their plane tickets to identify themselves as eligible for pre-check. As of 2015, 10 U.S. airlines participate in the program.

The TSA allows passengers considered low-risk travelers to apply for security pre-checks. This gives them expedited security screenings for domestic and international flights at participating checkpoints in the United States. As of 2015, a KTN is valid for five years. Passengers must use the same names on plane tickets that they used on their TSA pre-check applications.