Just the random number generator in the Vassal program that simulates rolling a D20. Technically they are Vassal 'dice' I guess.
In the Vassal program, when you're playing a SWM game online against someone, and you need to roll a die for an attack or a save or initiative or something, you just click the button, and it randomly kicks out a number between 1 and 20. Oddly, it seems that sometimes one person will get an average of higher rolls than the other person. It's really just a perceived thing though. As much as anyone might disagree, the Vassal dice are random, and shouldn't favor one player over the other. I know it seems that way though, sometimes. Heck, my real life dice seem to never favor me, lol.