1 / 12 means one full rotation every 12 inches. Suitable for shorter (and therefore usually lighter) projectiles. This is the ideal twist rate for 36 and 40 grain projectiles.
1x9 is most suitable for 55 grain projectiles.
1x8 and 1x7 are suitable for longer projectiles like 75 gr and 80 grain vlds.
I know of a chart that acts like a good guildeline to help you understand let me see if I can dig it up.
But please remember, twist rate is just one of MANY factors. A well made barrel being much more important than just twist rate. Consistency of rifling, an even crown, an in spec chamber, barrel profile and more must be considered.
The most important thing for accuracy in my humble opinion is as follows #1 shooter, #2 ammo, #3 gun. If your ammo is only capable of doing 3 moa, it doesn't matter if your gun is able to do 1/4 moa. If the shooter is only capable of 3 moa, the best gun and ammo ain't gonna change that, so practice practice practice.
Assumption is the mother of all ****ups.
Know what you know, Know what you don't know. -Paul Pedzolt
"Jack of all trades, master of none, though oftentimes better than master of one" -Mynshul