This simply refers to bandwidth. PC2100 is clocked at 133 MHz, and accordingly has twice the theoretical throughput of single data rate PC133, which is 1066 MB/s. So, the bandwidth of PC2100 is up to 2x1066 = 2133 MB/s. In the same way PC1600 is clocked at 100 MHz and has 2x800 = 1600 MB/s bandwidth.