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Bryan, from the factory, every car is designed so that the steering arms and a-arms move in carefully prescribed arcs. This ensures that the effective length of the steering arms stays the same as the suspension goes from compression to droop. So for example, when you stand on the brakes hard, weight transfers to the front of the car and the front springs compress considerably. The front wheels continue to point straight ahead.
On a significantly lowered car, the relationship between the arcs of the a-arms and steering arms is altered. As the suspension moves into compression, the steering arm becomes effectively shorter and the wheels toe out (on 105/115) cars. When you bump steer a car, you are moving the steering arc to correspond with the lowered a-arm arcs. This is typically done by replacing the outer tie rod with a rod end (heim). The rod end can be spaced down until bump steer can again be brought under control.
For the vast majority of street cars, this process is unnecessary. But if you car has been lowered 2 inches or more, it's a good idea!
Erik
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