A refresh cycle is an occasionally repeated procedure or cycle that the random access memory (RAM) undergoes. It reads and then rewrites the contents of a dynamic memory device. This is done to ensure that the information within the RAM will not disappear. Without this cycle, any information that the computer reads will disappear within a few milliseconds.
Techopedia explains Refresh Cycle
When a computer reads data, it stores the data as binary digits (0/1 or on/off). When an instance of "0," or "off" is made, it does not require any electrical charge. If, on the other hand, an instance of "1," or "on" is made, it requires electrical charge to be able to turn the electronic switches (memory) on and store the data. The electrically charged areas, and the off areas for that matter, are used to convey the information from the memory that the user requests. This charge is very minute and only temporary, and if it disappears it would turn all the ones in the RAM to zeros, and the information would disappear. To guarantee that the ones won't lose charge causing a loss of data, the refresh cycle is done to recharge the memory with the necessary electrons.
There are two types of RAM: Static Random Access Module (SRAM) and Dynamic Random Access Module (DRAM). Among the two types, DRAM does the cycle more often because it is used for the computer’s main memory. DRAM is constructed to hold each section of the read data in a separate capacitor, where each capacitor represents one memory bit. This structure makes the storage of information much easier, but it also makes it more unstable if the memory refresh cycle will not recharge the DRAM memories.