I've been playing since launch, and I don't get it either.
What level character are you playing?
If you're leveling, I wouldn't worry about it. Trying for optimal stats is expensive, and the benefits are short lived when leveling. For example, on a whim I bought a lvl 30 Cartel gear set for one of my alts. I was replacing the purple mods with blue in just a few hours.
Once you get into end-game content and raiding, it will make a difference.
Check out http://www.noxxic.com/swtor/
. It does a pretty good job telling what stats to look for when you're ready to "be all you can be".