AWS stands for Automatic Warning system. It's a device fitted to British trains since the 1950s to warn drivers that they're approaching a signal. Originally AWS alerted drivers only to distance signals set to caution but was later developed to include all signal aspects.
Earlier systems in Europe and on the GWR here in England had proved unsuccessful because they relied on physical contact between the locomotive and ground apparatus to trigger the warning making them unsuitable for higher speeds.
AWS is more successful because it uses a magnet placed on the track to trigger a receiver attached to the locomotive meaning no physical contact is required.
When a train passes over a magnet, the driver receives an audible warning in the form of a bell (for clear signals) or a horn (for danger or caution signals). The driver will also get a visual warning on a round-shaped in-cab display unit known as the sunflower because of its yellow and black appearance when showing caution. The driver has just a few seconds to press a button acknowledging the warning before the train's brakes are automatically applied.
AWS was a huge step forwards for railway safety, however, because it could be overridden by the driver it was not failsafe. Because of this AWS has been replaced by ATP (Automatic Train Protection) that will take control of the train should a signal be passed at danger.