在网上下载了一个源码，有几个不懂的地方，请教高人

[monong]

var slideNum = 7;

var frameTimer = 200;

var speed = 0.3;

var imgWidth = this.slideImg._width/slideNum;
/////////////////////////////
//
///////////////////////////////////////////////////////////////////////
//
var slideOver = 0;
function slideMove() {
this.slideImg._x += speed*((targetx-1)-this.slideImg._x);
}
function slideSystem() {
for (var i = 1; i<=slideNum; i++) {
if (slideOver == i) {
if (this["slideMc"+i]._currentframe < this["slideMc"+i]._totalframes) {
this["slideMc"+i].nextFrame();
targetx = -imgWidth*(i-1);
}
} else {
if (this["slideMc"+i]._currentframe>1) {
this["slideMc"+i].prevFrame();
}
}
}
}
MovieClip.prototype.imgRollOver = function() {
mouseOver = true;
};
MovieClip.prototype.imgRollOut = function() {
mouseOver = false;
};
MovieClip.prototype.btnRollOver = function() {
slideOver = this._name.slice(7);
mouseOver = true;
};
MovieClip.prototype.btnRollOut = function() {
mouseOver = false;
};
mouseOver = false;
this.onEnterFrame = function() {
slideMove();
slideSystem();
if (mouseOver == false) {
if (timer++%frameTimer == 0) {
if (slideOver == slideNum) {
slideOver = 0;
}
slideOver++;
}
}
};
问题：1、targetx = -imgWidth*(i-1); 为什么是 -imgWidt
2、MovieClip.prototype.imgRollOver = function() {
mouseOver = true;
};
MovieClip.prototype.imgRollOut = function() {
mouseOver = false;
};
MovieClip.prototype.btnRollOver = function() {
slideOver = this._name.slice(7);
mouseOver = true;
};
MovieClip.prototype.btnRollOut = function() {
mouseOver = false;
};这几句的“=”前面的内容是什么意思呢？
3、if (timer++%frameTimer == 0) 是什么意思呢？timer是从哪来的。
先谢谢了！！！