teach me about memory timing

[mustangless]

I am totally lost when it comes to memory timing. I hear that you need tight for good OC but dont understand that. With my value ram I have from cpu-z

Frequency 133MHz 166MHz
CAS# Latency 2.0 2.5
RAS# to CAS# 3 3
RAS# Precharge 3 3
TRas# 6 7


what does all of this mean? and why is it showing two different frequencies?

 

[DAPUNISHER]

here you go The reason it shows 2 sets is that those are the timings it is reated to run@the respective speeds of 133 and 166.