A memory divider is a ratio which is used to determine the operating clock frequency of computer memory in accordance with front side bus (FSB) frequency, if the memory system is dependent on FSB clock speed.
Ideally, Front Side Bus and system memory should run at the same clock speed because FSB connects system memory to the CPU, but it is sometimes desired to run the FSB and system memory at different clock speeds.
So, settings termed as Memory Divider or FSB/DRAM settings are available and are expressed in a "ratio" which control the difference in memory clock rate and FSB speed.
High end motherboards meant for overclocking provide facilities to change memory dividers (if chipset supports memory dividers).
Memory clock then determines the final operating frequency or effective clock speed of memory system depending upon DRAM types (DDR, DDR2 and DDR3 SDRAM).
To keep running overclocked CPU at 2.5 GHz or even at higher speeds (by increasing FSB) we need to slow down memory clock so as to achieve a stable system.