css 这段代码什么意思？小白求解答

Question

 .page_bg {

    position: absolute;
    top: 50%;
    left: 50%;
    width: 2500px;
    height: 1600px;
    margin: -800px 0 0 -1250px;
     z-index: -1;

}
.page_bg:before {
    content: "";
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: url(../img/bg1.jpg) no-repeat center center;
    transform-origin: center 708px;
    -webkit-transform-origin: center 708px;
}

Answer 1

绝对定位，中心对齐，底层。