The blocks of four figures refer to the "latency " of the memory module...the following will give you some idea of the meaning....As with almost all latency issues, the lower, the better. When picking out SDRAM to buy, these four measurements are only important in high performance computers. RAM speeds are given by the four numbers above, in the format "tCAS-tRCD-tRP-tRAS". So, for example, latency values given as 2.5-3-3-5 would indicate tCAS=2.5, tRCD=3, tRP=3, tRAS=5. (Note that .5 values of latency (such as 2.5) are only possible in Double data rate RAM, where two parts of each clock cycle are used)
Most computer users don't need to worry about SDRAM latency, because the computer can handle the auto-adjustment to RAM timing based on the ROM inside the RAM packaging that defines the four timing values, decided by the RAM manufacturer. Although the SDRAM latency timing can be adjusted manually, using lower latency settings than the module's rating (overclocking) may cause a computer to crash or fail to boot.