A "flat tire" occurs when a vehicle's tire loses air pressure, making it unable to support the weight of the vehicle. This can happen due to various reasons, such as a puncture from a sharp object, a slow leak, or damage to the tire itself. When a tire is flat, it can cause difficulty in steering and may lead to further damage if driven on. To fix a flat tire, drivers often need to replace it with a spare tire or repair it using a patch or sealant. It's important to regularly check the air pressure in tires to prevent flats and ensure safe driving. Proper maintenance of tires can help extend their lifespan and improve vehicle performance.