In the How-To in the bottom of my signature, the 2nd post covers polishing. Basically, all your doing is putting a better finish on the parts that contact/rub to make them smoother. Some parts from factory are kinda rough, this helps smooth and get some of the stacking out of the trigger pull. It doesn't normally lighten the pull, if it does, it's not a noticeable amount (IMO).
Previously my SR9 trigger stacked kinda like this when pulling the trigger:
.._._.. ..._.bang.
Now it's pretty much just: ______bang.
Just take a dremel with buffing wheels and some Flitz (works best to most, I couldn't find any locally) and take your time on the parts. Since your using a fine polishing compound and cloth wheels, it's really hard to really deform the metal, but make sure you follow the contours and shapes of the metal and don't change any profiles. You'd need something with real grit to actually do that, but it's just a precautionary thing we always mention.
Check the How-To for more formal instruction.
Josh