In simple terms, understeer refers to the car turning less than the amount of steering the driver applies in a corner. Between understeer and oversteer, understeer is less dangerous since it is more predictable and, therefore, easier to correct.
Understeer is a phenomenon that predominantly happens with front-wheel-drive cars or AWD configurations with a front-biased axle. In FWD cars, front wheels have a lot of tasks they need to handle simultaneously. They control the vehicle’s acceleration and direct it in turns.
When entering a corner with excessive speed and acceleration, the front tires lose grip, resulting in less traction with the road. Because of this, the front tires slide over the road surface and lead the car in a straighter than desired line. How much speed is too much for a corner heavily depends on your car, the corner angle, and the road quality. Even 20 mph to go around a square on a rainy day can be too much for some FWD cars.
A similar scenario can happen when you brake too hard when entering a corner in a FWD vehicle. The sudden decrease in acceleration can make the front tires slide on the road without being able to maneuver the car in the desired direction.
