Inflating one's individual score or statistics in a video game without productive contribution to gameplay. Usually refers to a style of play that neglects all other activities except those yielding easy points to personal score.
In games requiring teamwork, the "padder's" contribution to the team effort and their team's capabilities usually suffer as a result.
One probable origin of the expression is the video game Battlefield 2, where various, originally supportive activities could be abused to gain individual score without actually contributing to gameplay. In Battlefield: Bad Company 2 the tradition continued, as reviving dying players repeatedly with the defibrillator "pads" yielded relatively high personal score, but in most instances contributed little to the team effort, as the medic would essentially abandon all other activity, sometimes even being counterproductive by preventing teammates from playing freely.
*a round ends*
padder> "OH YEAH #1"
player> "we lost you statspadding
dick, you cost us 30 tickets for your 2 kills"
padder> "YEAH BUT I AM THE BEST"
player> "no, you're just a score whore
*player has joined the other team*
Red1> "god I hate stat padding
Red2> "same, I just got revived three times in grenade spam
Red1> "I know right, you're just stuck there, dying over and over again because some asshole keeps reviving and not shooting the enemies"