If your car has a low windshield wiper fluid light, it means that the level of washer fluid in the reservoir is getting low and needs to be refilled. Depending on your car, the light may come on when there's only enough washer fluid left for one or two wipes. Or, it may come on when the reservoir is almost empty. Either way, it's important to refill the reservoir as soon as possible so you don't run out of washer fluid while driving.
If you do run out of washer fluid, you can still use your wipers, but they won't work as well because there's nothing to help remove the water and debris from your windshield. This can make it difficult to see while driving, which can lead to accidents. So if you see the low windshield wiper fluid light, be sure to stop at a gas station or auto parts store and fill up the reservoir right away.