Ruger was sued over this issue, so all pistols sent in get the transfer bar mod done. As far as operation you won't notice anything in operation. They also send you the original parts. The only permanent alteration is a stamp under the grip panels.
Before sending it in, check your hammer face to see if it's notched for the bar.

Basically, the transfer bar is attached to the trigger. If the hammer drops for whatever reason (without the trigger being pulled) the pistol cannot go off as the upper lug of the hammer stops the hammer from hitting the firing pin as it impacts the back of the frame first. When the trigger is pulled, the transfer bar is forced up ad the notched part of the trigger contacts it, which hits the firing pin.