An electronics term.
There are two types of RAM - Static RAM and Dynamic RAM.
Dynamic RAM needs a constant stream of electric pulses to keeps its data, if the pulses were stopped it would almost instantly lose all its data. This set of pulses is called the refresh signal.
Static RAM chips do not need a refresh signal but they are more expensive than dynamic RAM. So cheap dynamic RAM chips are used for main memory whilst the faster and more expensive static RAM chips are used for cache.
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Click on this link: Refresh signal