A 'Hertz' is another word for 'cycles per second'. It is a measure of frequency and computer clock speed.
'Giga' is the metric term for one thousand million.
So 1GHz is 1 thousand million (or one billion) cycles per second
For example a computer running at 1GHz means that its internal clock is 'ticking' a thousand million times per second.
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Click on this link: GigaHertz