What does the warning light exclamation symbol mean on the dashboard of your Scion tc?

It means your tire pressure is low on one of your tires. Fill to the specified psi (located on the tire wall), then push reset button to turn off exclamation point light (located in glovebox). Owner's manual tells you all about it.

The pressure listed on the tire sidewall is the max safe pressure for that tire. The pressure specification for the vehicle will be listed on a label on the drivers door jamb, in the owners manual, or in the glove box.