Because a coil spring is tapered, it has to mate up with something else that is tapered in the opposite direction. This keeps the weight of the car in contact with the entire top and bottom coil of the spring instead of just one point on the top and one point on the bottom. Typically springs fit into a tapered piece (spring stop) that has a surface tapered the same way as the spring, and a flat edge for the spring to fit against, this keeps the spring from rotating and distorting.

It really is a hard thing to describe, but if you do decide to alter your springs just be sure to take really good pictures before you do anything. This will help in re-assembly!

As soon as you have the spring out you'll see exactly what it mounts into...Then it'll hit you like a ton of bricks and you'll go "THAT is what they were talking about!"