is the speed, in MHz, that it is clocked at. SDRAM and DDR SDRAM, and DDR2 SDRAM are all forms of Synchronous Data Random Access Memory. The synchronous refers to the fact that they only transfer data in synchronism with a clock signal.
For years the memory speed was 66 MHz, then they came out with the faster, 100 MHz, 133 MHz, 166 MHz, 200 MHz and then the same with DDR [double data rate] memory whichs transfers data at both the rise and the fall of the clock signal, thus can be referred to as 200, 266, 333, 400. Now there is DDR2 memory which is clocked at higher rates yet.
for DDR: PC 1600 is DDR200, PC 2100 is DDR 266, PC 2700 is DDR 333, PC 3200 is DDR 400, PC 3700 is DDR 466, PC 4000 is DDR 500, PC 4400 is DDR 550, PC 4800 is DDR 600, PC 5600 is DDR 700. The actual clock speed of each type is half of the DDR number. The PC number is basically the memory bandwidth.
The clock speed of the memory is important, because it basically determines the speed of the FSB, which in turn determines the speed that the CPU runs at internally. [these relationships are not exactly directly related in the later CPU's, it gets fairly complex].
If we ignore the subject of overclocking, if you have a mix of different speed memory sticks in a system, they will all be clocked at the speed of the slowest. WHY? Simply because it is synchronous memory and can only run in synchronism with the memory clock. If you were to clock it faster than the speed of the slowest memory, the memory would be erratic and the computer will be very unstable, and may not even run.