Bump steer is a handling problem. It occurs when a tire hits a bump and the suspension compresses, forcing that wheel to steer left or right on its own. This happens because the arc of the tie rod does not match the travel path of the control arm. As the wheel moves up and down, the tie rod pulls or pushes on the steering knuckle, changing the wheel angle. The driver has to constantly correct the steering wheel on bumpy roads.
This is common on lifted trucks and lowered sports cars. Modifying the ride height shifts the steering rack and control arms out of their factory alignment angles. Correcting bump steer requires installing adjustable tie rod ends (bump steer kits) or dropping the steering rack to restore the parallel angle between the tie rods and control arms.