The fractal controls the blend between the low colour and high colour. Imagine the fractal produces values ranging between 0 and 1. Where 0 is produced, the shader outputs the low colour. Where 1 is produced, the shader outputs the high colour. However, the fractal is controlled by the "colour contrast", "colour offset" and "colour roughness" parameters. Especially "colour contrast" and "colour offset", these can push the fractal outside the range 0 to 1. The simplest way to test this is to set contrast to a high value e.g. 10. When this happens, colours will be extrapolated outside the range of the low colour and high colour. If these colours are simply white and black (with values 1 and 0), you can easily predict the outcome. You'll get greyscale colours that are brighter than white (greater than 1) and on the low end you'll get negative colours. With other colours, mathematically the same extrapolation occurs, but figuring out what colours will be produced is a bit less intuitive.
The clamp options allow you to clamp the fractal values to the range 0 to 1 so that the output colours never extrapolate outside of the mix between the low colour and the high colour. So, it does not clamp the output colours to black and white - it clamps at the colours specified in by "low colour" and "high colour".
Unfortunately the explanation in the Wiki is wrong. I'll have to fix that.
Matt