When shopping for computer RAM you'll likely notice that in the specifications for each is a CAS (Column Access Strobe) Latency number, like CL16 on many 2666MHz DDR4 RAM kits.
Basically, the CAS latency, or CL, is an indication of the time it takes for your memory controller to get a particular piece of data ready to access from the RAM, so lower numbers are generally more desirable.
It's important to note this number appearing after CL, as in CL16 for example, is just one part of the whole process, and the measurement of time in these processes are NS (nano seconds) being used, which is unit of time equal to one thousand-millionth of a second.
To expand on this a little more, without going too deep, if I look at 2666MHz CL16 DDR4 vs 2800MHz CL16 DDR4 the 2800MHz kit is faster, but it's going to be hard to notice as they are both incredibly fast.
On the PB Tech Website, when you are shopping for RAM, once you choose the type (Desktop RAM, Laptop RAM or Server RAM) you can filter the results as shown below to find specific CL, MHz, sizes and more.