z-index

* {
 margin: 0;
 padding: 0;
}

.main {
 height: 500px;
 width: 800px;
 position: absolute;
 top: 50%;
 left: 50%;
 margin-top: -250px;
 margin-left: -400px;
 border: 1px solid #808080;
}

#div1, #div2, #div3, #div4, #div5 {
 height: 100px;
 width: 300px;
 border: 1px solid #808080;
 position: relative;
} 

#div1 {
 top: 0;
 left: 0;
 background-color: rgba(59, 169, 222, 0.5);
 z-index: 1;
}

#div2 {
 top: -50px;
 left: 50px;
 background-color: rgba(59, 169, 222, 0.5);
}

#div3 {
 top: -100px;
 left: 100px;
 background-color: rgba(59, 169, 222, 0.5);
}

#div4 {
 top: -150px;
 left: 150px;
 background-color: rgba(59, 169, 222, 0.5);
 z-index: 4;
}

#div5 {
 top: -200px;
 left: 200px;
 background-color: rgba(59, 169, 222, 0.5);
 z-index: 5;
}