What's the purpose of the :before property in this code? - css

Question

CSS newbie here.
I was trying to understand the code to get this parallax effect but I still can't understand why the user decided to put ":before" on two out of four divs.
I read that :before is used to attach something before an element but I can't understand what is being attached here. Can someone help me? Thanks.
https://codepen.io/keithclark/pen/JycFw
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#import url(https://fonts.googleapis.com/css?family=Nunito);
html {
height: 100%;
overflow: hidden;
}
body {
margin:0;
padding:0;
perspective: 1px;
transform-style: preserve-3d;
height: 100%;
overflow-y: scroll;
overflow-x: hidden;
font-family: Nunito;
}
h1 {
font-size: 250%
}
p {
font-size: 140%;
line-height: 150%;
color: #333;
}
.slide {
position: relative;
padding: 25vh 10%;
min-height: 100vh;
width: 100vw;
box-sizing: border-box;
box-shadow: 0 -1px 10px rgba(0, 0, 0, .7);
transform-style: inherit;
}
img {
position: absolute;
top: 50%;
left: 35%;
width: 320px;
height: 240px;
transform: translateZ(.25px) scale(.75) translateX(-94%) translateY(-100%) rotate(2deg);
padding: 10px;
border-radius: 5px;
background: rgba(240,230,220, .7);
box-shadow: 0 0 8px rgba(0, 0, 0, .7);
}
img:last-of-type {
transform: translateZ(.4px) scale(.6) translateX(-104%) translateY(-40%) rotate(-5deg);
}
.slide:before {
content: "";
position: absolute;
top: 0;
bottom: 0;
left: 0;
right: 0;
box-shadow: 0 0 8px 1px rgba(0, 0, 0, .7);
}
.title {
width: 50%;
padding: 5%;
border-radius: 5px;
background: rgba(240,230,220, .7);
box-shadow: 0 0 8px rgba(0, 0, 0, .7);
}
.slide:nth-child(2n) .title {
margin-left: 0;
margin-right: auto;
}
.slide:nth-child(2n+1) .title {
margin-left: auto;
margin-right: 0;
}
.slide, .slide:before {
background: 50% 50% / cover;
}
.header {
text-align: center;
font-size: 175%;
color: #fff;
text-shadow: 0 2px 2px #000;
}
#title {
background-image: url("https://lorempixel.com/640/480/abstract/6/");
z-index:2;
}
#title h1 {
transform: translateZ(.25px) scale(.75);
transform-origin: 50% 100%;
}
#slide1:before {
background-image: url("https://lorempixel.com/640/480/abstract/4/");
transform: translateZ(-1px) scale(2);
}
#slide2 {
background-image: url("https://lorempixel.com/640/480/abstract/3/");
z-index:2;
}
#slide3:before {
background-image: url("https://lorempixel.com/640/480/abstract/5/");
transform: translateZ(-1px) scale(2);
}
#slide4 {
background: #222;
}