The cpu clock is used to keep everything in the CPU happening in sync. It's kind of like the drummer in a band as if he slows the tempo then the band slows down. If he speeds up the band speeds up until they can't keep up and it falls apart.
A clock runs in a a cycle...from a low to a high voltage back to low. The clock is seen as pulses, going from a low to a high voltage if you had an oscilloscope.