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:
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.
first, you are not really usually polishing the trigger but the sear and hammer parts inside the trigger group... now this may be the top end of the trigger but not the part most folks would call the trigger.
As mentioned above it is to get a lighter and smoother trigger pull and hammer strike.
If you've ever shot a rifle or even a pistol from a bench rest and tried to hold the sights absolutely still and squeeze the trigger then you will find that most triggers are just plain awful. The problem is that folks having or doing these themselves will sometimes miss that the manufacturer made them that way for a reason. A very light and easy trigger pull might be great for target shooting but it can be very dangerous for self defense. There is a good reason why military rifles have a heavy trigger pull.