Well if u are familiar with Skate Boards or roller blades they have bearings, most of the time roller bearings. These are little metal balls(most of the time) that sit in a circle.
Here is an image of what they can look like. Just to give u an idea http://www.made-in-china.com/servlet...d=hTEtoFzMOaWJ
What they do is help make the trigger pull(in this case) smooth and glide easier instead of the friction of just metal on metal. They minimize the friction between to parts to make it smoother and easier to move.
Hope that helps, if anyone wants to add anything just go for it.