those are latencies, which are "clocks per cycle". It's basically the number of clock ticks between each operation. The most important one is the first one: from 1.5, 2, 2.5, 3 etc. The lower the better, but 3 isn't twice as good as 1.5. They affect performance on every machine, but the higher the latency is, the faster the chips can be run. Typically, overclocking the chip (I.E. running memory at 250MHz instead of 200MHz) results in a performance increase greater than the negative impact of the higher latency.
So, if you can run your memory up to 200MHz with 2.5 3 3 8, and it is also stable at 250MHz with 3 4 3 8, you will most likely see an overall performance increase, even though the timings are less desirable.