Easy to explain but much more difficult to get a visual picture of and try and understand but I will try
Clock frequency is simply the measurement of how quickly a processor can perform an operation, and an operation within a processor is the process of carrying an instruction set down a pipeline from point A to point B meeting a number of stages while travelling down that pipeline that allow the processor to use the instruction set
Therefore, one can define 1Hz as the time it takes x to travel from A to B. Now, assume you have a Pentium 4 Prescott and an AMD64, both use pipelines to process instruction sets. I believe the AMD64 has 12 pipeline stages versus the prescott that has 30 stages but don't quote me on those numbers...it is evident the Pentium has much longer pipelines than the Athlon
If you were to look back at the basic formula, you would see that 1Hz on an AMD would not be equal to 1Hz on an Intel considering the pipeline length...the AMD is able to execute the instruction set much faster than the Intel therefore it requires less number of Hz to perform similar tasks