The phrase "to alert the driver" means to notify or warn the person who is driving about something important or potentially dangerous that they need to pay attention to. Full definition
At speeds above 18 mph, the system alerts the driver of a vehicle in the blind spot or approaching quickly from behind via a visual signal from the exterior mirrors. (porsche.com)
Using a combination of radar mounted behind the car's front grille, and digital cameras located under the rear - view mirror, the system first alerts drivers when it detects pedestrians. (macleans.ca)
The system calculates the car's speed and trajectory and if there might be a problem looming ahead, then alerts the driver if he needs to change course or slow down. (cnet.com)