Memory latency? What does it mean

[HibikiRush]

I am thinking about purchasing some Corsair XMS RAM, but I see they have different types. C2, LL, and some others. It seems one of them has lower CAS latency (2-2-2-5-T1 compared to 2-3-3-T1?). What does CAS latency do, and how does it affect performance? Should I add the extra couple of bucks and get the better RAM?