Automatic Warning System, or AWS for short, is the orange/yellow exclamation mark that appears on the right side of the Driver's GUI, which indicates to the driver that he/she needs to slow down because there is a red/yellow signal ahead. In the real world AWS is also sometimes used to warn the driver of an upcomming speed reduction. You need to either press the orange/yellow exclamation mark or hit 'Q' on the keyboard within 6 seconds after the AWS starts to beep. Failing to confirm the AWS will result in an emergency brake, like the one you get after you SPAD. You can be ejected if you wait too long to release the brakes.
As of now, the AWS does not warn you for speeding, but this may change in a future update.
Previously, the AWS in game worked very realistically, however, the AWS was recently changed to clear itself if the signal changes to green. This does not happen in the real world. The ingame AWS also sounds the alarm if the signal changes to a red. This also does not happen in the real world.