If you open up a ship's details and go to the components tab, and you hover over a weapon component, it has a rated accuracy. The +2 accurate trait increases the the rated accuracy by 0.25, the +1 accuracy trait increases the rated accuracy by 0.15, the -1 accuracy trait decreases the rated accuracy by 0.15, and the -2 accuracy trait decreases the rated accuracy by 0.25. The accuracy numbers are displayed in the game tool tips of weapon components as a percentage, so lasers (which have a basic accuracy of 1) will have a rated accuracy of 125%/115%/100%/85%/75% with Accuracy +2/+1/0/-1/-2.

I can only assume that this is the base hit chance of the weapon and that this is in some way modified by jamming and the tactical speed of the target, but how I don't know. At a guess, the jamming bonus is subtracted from the modified accuracy (so if you have the +1 accuracy trait and you're going up against someone with the +1 agile trait, these bonuses should cancel one another out) and then tactical speed comes into play somehow. However, that's a guess; I don't really know how accuracy, jamming, and tactical speed (and any other related attributes) interact. I do know that GlobalDefs defines a minimum hit chance of 10%, so that at least gives you an upper limit on how much jamming can do for you and how badly you can be hurt by it.