Its the speed that your memory modules run at,
533MHz means it cycles at 533 million 'clocks' per second, Google memory MHz for more technical info, I C&P'd this much
*Stolen from webpage*
Memory RAM Speed - Access Time, Megahertz (MHz), Bytes Per Second
Prior to SDRAM, speed was expressed in terms of nanoseconds (ns). This measured the amount of time it takes the module to deliver a data request. Therefore, the lower the nanosecond speed, the faster. Typical speeds were 90, 80, 70 and 60ns. Older 486 machines may have 80 or 90. More recent Pentiums will have 60 or 70.
MHz Speed Total Clock Cycles per Second Divide by 1 billions to get nanoseconds per clock speed.
66 66,000,000 15
100 100,000,000 10
133 133,000,000 8
Often, the last digit of a memory part number will represents the speed such as -6 = 60ns.
SDRAM speed is measured in megahertz (MHz). Speed markings on the memory chips may still specific nanoseconds, but in this case in represents the number of nanoseconds between clock cycles. To add to the confusion the markings on the chips don't match the Mhz value. Here is a conversion chart.
To calculate bytes per second you need to know the Bus Width and Bus Speed of your PC. The first thing to remember is 8-bits = 1 byte. If you have a 64-bit bus, than 8 bytes of information can be transferred at one time. (64 / 8 bits = 8 bytes)
If your bus speed is 100Mhz (100 million clock cycles per second) and the bus width is 1 byte wide, the speed is 100 MB's per second. With a 64-bit width, the speed is 800 MBs per second (64 / 8 * 100,000,000)
Rambus modules are measured in megabytes per second. Rambus modules are either 400 or 300Mhz. Because they send two pieces of information every clock cycle, you get 800 or 600Mhz. They have a 16-bit bus width or 2 bytes (16/8). The 400Mhz module speed is 1600MB a second or 1.6GB a second. (400,000,000 * 2) * 2. The 300Mhz module provides 1.2GBs a second
Hope this helps,