The MHz number tells the clock speed of the RAM - the number of cycles it performs in once second. The CL number, or CAS Latency number, is, in a very basic sense, the number of cycles that pass between the RAM getting a command to access a specific data, and when the data actually becomes available. Basically, you can say it is the time between the RAM getting a command, and executing this command. Again, this is a very vague explanation, you can of course see it in more technical terms on any website with a quick Google search.
Also, lower the CAS Latency, faster is the RAM.