The numbers are a measure of the time (measured in RAM cycles), technically termed latency, that it takes for the RAM to retrieve a piece of data (the different numbers measure timings between different points in the retrieval process).
Essentially, if you have 1333 MHz RAM - the RAM can transfer data at 1333 MHz. However, that speed can only be achieved during a transfer batch. There is latency (delay) from when the RAM receives a request, and when the RAM actually starts delievering the data.
For example, the first 9 in 9-9-9-24, is CAS latency (CL) which means that after the motherboard sends the final part of the signal requesting data from RAM (CAS) there is a delay of 9 cycles, before the data becomes ready.
The other numbers measure different timings: the 2nd number is RAS-CAS delay, which is how long the delay must be between the motherboard sending the first part of a request (RAS) and the 2nd part (CAS). the 3rd number is RAS precharge delay, and is the recovery time the RAM needs before it can accept a new RAS command. The last number is a total timing (the amount of time needed for a request to go through, and then the RAM to recover to the point where it is able to take another command straight away).
The most important is CL (the first number) as every memory access requires a CAS signal. RAS is less important, as RAS commands can be reused.
So, lower is better. However, it's more subtle than that. The actual time on a CL9 1600 MHz module is 9/1600000000 seconds = 5.6 ns, whereas the time on a CL8 1333 MHz module is 6.0 ns. (The latency number is lower, but because the clock speed is lower, the actual latency time is higher).
As it is, the CPU cache performs very efficiently in hiding the memory latency - reducing it by about 95%. This means that memory latency has virtually no effect on system performance. The maximum you are likely to get from extreme tweaking is 1-2%.
Not only that, but the latencies are the most difficult thing for the RAM, and tweaking them successfully often needs overvolting RAM (e.g. DDR3 is 1.5V - however, many manufacturers overvolt the RAM to 1.65 in order to get the latency down. This seriously degrades the reliability and lifespan of the RAM - and I, for one, will never buy 'overvolted' RAM again, because I had so much of it burn out very quickly)