Could somone explain it to me?

programming flash
1

MovieClip.prototype.changePhoto = function(d) {
this.pIndex = (this.pIndex+d)%this.pArray.length;
if (this.pIndex<0) {
this.pIndex += this.pArray.length;
}
this.onEnterFrame = fadeOut;
};

Could someone be so kind and explain this to me…
“pIndex” starts al zero (0)

and what the $&$% is “this.pArray.length”