<div class="box box1">Позади</div>
<div class="box box2">Поверх</div>

<style>
  .box {
    position: absolute;
    width: 200px;
    height: 100px;
    color: white;
    padding: 10px;
  }

  .box1 {
    background: #888;
    top: 50px;
    left: 50px;
    z-index: 1;
  }

  .box2 {
    background: #e74c3c;
    top: 80px;
    left: 80px;
    z-index: 2;
  }
</style>