CPU chips have a clock input, which controls the duration of a cycle. A cycle is just a state transition within the CPU's internal circuits. Each instruction the CPU executes requires one or more cycles. Any given CPU will have a rated clock speed, meaning that the manufacturer guarantees the results will be reliable at that speed. You can try to increase the clock speed (overclock the CPU), but it is possible that in some cycles the circuits within the CPU won't have fully completed their transition to the next state. So, you might get unreliable results. Working the computers circuits faster will also require more power, generate more heat, and lead to timing problems with external components. In short, it is a way to make your computer faster and less reliable.