Most integrated circuits of sufficient complexity require a clock signal in order to synchronize different parts of the chip and to account for gate delays. As chips get more complex, the problem of supplying accurate and synchronized clocks to all the circuits becomes more and more difficult. The preeminent example of such complex chips are microprocessors, the central part of modern computers.
The clock frequency of a microcomputers CPU is very often used as a primary measure of the computers performance. CPU clock frequency is traditionally measured in MHz, but improved technology has moved this into the GHz range. Note that many factors affect computer performance, the clock frequency is only one of them.