It's an extra security step on flights to the USA - Secondary Security Screening Selection.

It's supposedly random, and will be stamped on the printed boarding pass issued at the airport. Then at the gate, they will take you downstairs to a little table, and swab your hands, shoes, belongings etc. It's a hassle but only a small one. A good tip, if travelling with a companion, is to give them your hand luggage so there is less to be checked.