As CNN explained, the Transportation Security Administration randomly swabs passengers' hands at checkpoints and airport gates to test them for traces of explosives. This was an expansion from simply swabbing luggage and other items.

Why does TSA pat me down?

A pat-down is an additional security precaution used by TSA to determine if a traveler is concealing something prohibited on their person. ... Others may be pulled out of line if they have a certain sticker on their passport or if they happen to be acting suspicious – TSA is trained to catch strange behavior.

What does TSA swab test for?

The purpose of the test is to check for chemicals that might be used as explosives. The test can't check for all the chemicals that might be used by terrorists, so it looks for two sets of compounds that can be used to make many types of bombs: nitrates and glycerin. The good news is the test is highly sensitive.