If you see this mysterious code on your boarding pass, get ready to be searched

There are few things more dreaded in US airports than a long, snaking security line. But, if you see the letter "SSSS" printed on the bottom of your boarding pass, expect to spend some extra time with the fine people at Transportation Security Administration (TSA).

That's because the passengers whose boarding passes bear these four letters have been pre-selected for additional security screening.

"SSSS stands for Secondary Security Screening Selection and it appears on a passenger's boarding pass when they've been selected by TSA's Secure Flight system for enhanced security screening," a TSA spokesman told Business Insider in a statement.

According to the TSA, the meaning of "SSSS" has been public knowledge for years. However, the average traveler would be hard pressed to find reliable information on the mysterious four-letter designation without approaching the TSA.

In that same statement, the TSA explained the system used to select passengers for secondary screening.

"Secure Flight is a risk-based passenger prescreening program that enhances security by identifying low and high-risk passengers before they arrive at the airport by matching their names against trusted traveler lists and watchlists," the agency said.

Secure Flight collects only basic personal information like full name, date of birth, and gender, "necessary to conduct effective matching," the TSA told us. In addition, Secure Flight is also used to prevent those on the No Fly List and the Centers for Disease Control's Do Not Board List from getting on planes.