Bump steer is the tendency for a tyre hitting a bump to momentarily change its toe-in setting, which causes the car to feel like someone turned the steering wheel a bit. This happens due to the tyres’ suspension going up and down over the bump. The steering arm is attached to the body of the car at one end, and the suspension at the other end. So it travels in an arc. When a rod with one anchored end travels in an arc, the effective length of the rod changes. This is what pulls on the wheel and makes it feel as though someone besides the driver is steering the thing.
The 'Bumpsteer' modification effectively stops this from happening by adding shims to the steering rack, lessening the effect on the geometry.
Falkland Performance in Glenrothes did mine.