Most newer cars have a tire pressure monitoring system (TPMS) that uses sensors to monitor the air pressure in your tires and warn you when they get too low. The TPMS sensor is mounted on the wheel inside the tire, so it's not visible from outside the car.
When the TPMS sensor detects that tire pressure is getting low, it sends a signal to a warning light on the dash. The light looks like an exclamation point inside a U-shaped outline. If you see this light, it means you need to add air to your tires.
Not all cars have TPMS, though. Some older cars have a low-pressure warning light that comes on when any of the tires get low, but it doesn't tell you which one is low. And some cars don't have any kind of warning at all. So it's always a good idea to check your tire pressure regularly, even if your car has TPMS.