Gigahertz is the unit of a frequency. Frequency is the number of complete cycle per unit time. Imagine you are going around something, lets say a pole, and you take 2 seconds to make complete cycle around it, ie you go 1/2 round in per second. So your frequency is 1/2 per second. Per second (1/second) is Hertz. The term Hertz is coined after a German scientist Heinrich Rudolf Hertz,who worked in transmission and reception of electromagnetic waves.
Then you can make a standard conversion, mega = 10^6 (100000) Hertz. That means you go 100,000 cycle per second. Giga hertz is 100 Hertz or 10^9 (100,000,000) or Billion Hertz. Terra Hertz is the next higher unit. This is again 1000 Giga Hertz (10^12. or 1 trillion) Hertz. Further larger frequency prefix are peta (10^15 or 1 Quadrillion Hertz), exa (10^18 or 1 Quintillion), zetta (10^21 or 1 Sextillion), yotta (10^21 or 1 Septillion), each of which is 1000 times greater then the previous one.
In the smaller side we have Milli, micro, nano, pico, femto, atto, zepto, yocto. Which are again goes in the same order as in larger side, as 10 to the power -3,-6,-9,-12,-15,-18,-21 and -24, each 1000 times smaller then previous one.
In larger side, the frequency of visible light, you have to think of light as a wave, has frequency or order of 10^14 Hertz. The ultra violet ray, x-ray, gamma ray have higher and higher frequency then that of visible light. Hence they are more energetic. The infrared, telemetry, microwave, radar, television, FM radio and AM radio with medium wave and short wave have lower and lower frequency then visible light wave frequency. If you remember FM radio are broadcast around the 100 Mega Hertz.
Computer works with different logic. Digital logic either pass the information or not. That means they can have value either 0 or 1. If you computer can perform larger logic operation per second, that is better. In these day computer with the clock speed, ie the speed with which computer can switch from 0 to 1 or vica versa, of around a Giga hertz are available. If you want the computer work faster you need to look for the computer with the larger clock speed.
Now,if you go to office or school and comeback home for launch and again go to office and comeback home in the evening in a day,What do you think your frequency is ?