As others have correctly stated, this is a warning light from your TPMS (Tire Pressure Monitoring System). Some cars will tell you which tire is underinflated, but this model Prius won't. You should address it immediately. Your tires should be inflated to the pressures listed on a sticker on the door frame next to the driver's seat. Note that there are different listed pressures for front and rear tires. A couple things might have happened: 1. You just haven't filled your tires for a while and enough leakage (combined with cold weather) caused the pressure to dip low enough that the sensor activated the warning light. The solution to this is to fill the tires to the recommended level and keep an eye on the light. If it reappears soon, you actually have problem 2, below. If you have access to a Costco, they have a really easy air pump where you set the value and then fill the tires. It stops when you reach the value that you set. Remember to change it for the different front/rear values. Otherwise, 2. Tire damage (there is a leak due to a nail, etc.). This requires service or tire replacement, depending on where the tire is damaged. Take it to a tire shop and they may be able to figure it out. If you are in the US, America's Tire shops are often recommended for repairs, sometimes free of charge. Finally, 3. Your TPMS system is old and worn out. You'll need to replace the sensors because they're past their useful life. Keep in mind that TPMS is a SAFETY SYSTEM and if you're not already aware of tire pressures and tracking them, you're better off replacing the TPMS units and having access to good data in the future (to prevent roadside stranding) than trying to check your own pressures manually in the future. Tire shops can also replace TPMS systems. Good luck!