e.clientX中的e代表什么

document.onmousemove = function(e){
        //console.log("e.clientX:"+e.clientX);
        //console.log("coolPlay.offsetLeft:"+coolPlay.offsetLeft);
    var leftV = e.clientX - coolPlay.offsetLeft;
        //console.log("coolPlay.offsetLeft:"+coolPlay.offsetLeft);
        //console.log("leftV:"+leftV);
    if(leftV <= 0){
      leftV = 0;
    };
    if(leftV >= coolPlay.offsetWidth){
      leftV = coolPlay.offsetWidth-10;
    };
    cDrag.style.left = leftV+"px";
        //console.log(leftV);
};

};