SSSS, or Secondary Security Screening Selection, means you must now go through an enhanced screening process.
“Designed by the US's Transportation Security Administration (TSA) for flights using American airspace, the SSSS tag means that passengers are also limited in their airport habits - they can not print out their boarding passes at home (they will receive an error message) nor check in at a kiosk; they must visit a counter, where their identification can be verified," The Telegraph London explains.
While you can still fly, certain requirements must first be met.
So how do you end up on the SSSS list?
There are a couple of ways your name can be marked. First, if you’re on the government’s no-fly list. Or, the Centres for Disease Control and Prevention's Do Not Board List.
Other reasons could include passengers who have booked a one-way flight, passengers who pay for their ticket in cash and random selection.