Air travel can be stressful enough with tight schedules, packed airports, and long lines. The last thing you want is unexpected delays at security. But if you spot “SSSS” on your boarding pass, prepare for extra screening. Those four letters mean the TSA has selected you for additional checks. It’s an added step that can catch you off guard if you’re not expecting it.

“SSSS” stands for Secondary Security Screening Selection. It signals that you’ll go through more detailed security before boarding. This is part of the TSA’s Secure Flight program, which compares passenger information with watchlists.