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Please take a look at this code .. the ".active" class isn't in any other the element but it was declared in the css and JavaScript. How does it work?

<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>
.accordion {
background-color: #eee;
color: #444;
cursor: pointer;
padding: 18px;
width: 100%;
border: none;
text-align: left;
outline: none;
font-size: 15px;
transition: 0.4s;
}

.active, .accordion:hover {
background-color: #ccc;
}

.panel {
padding: 0 18px;
display: none;
background-color: white;
overflow: hidden;
}
</style>
</head>
<body>

<h2>Accordion</h2>

<button class="accordion">Section 1</button>
<div class="panel">
<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.</p>
</div>

<button class="accordion">Section 2</button>
<div class="panel">
<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.</p>
</div>

<button class="accordion">Section 3</button>
<div class="panel">
<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.</p>
</div>

<script>
var acc = document.getElementsByClassName("accordion";
var i;

for (i = 0; i < acc.length; i++) {
acc[i].addEventListener("click", function() {
this.classList.toggle("active";
var panel = this.nextElementSibling;
if (panel.style.display === "block" {
panel.style.display = "none";
} else {
panel.style.display = "block";
}
});
}
</script>

</body>
</html>

BabyApple:
Please take a look at this code .. the ".active" class isn't in any other the element but it was declared in the css and JavaScript. How does it work?

<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>
.accordion {
background-color: #eee;
color: #444;
cursor: pointer;
padding: 18px;
width: 100%;
border: none;
text-align: left;
outline: none;
font-size: 15px;
transition: 0.4s;
}

.active, .accordion:hover {
background-color: #ccc;
}

.panel {
padding: 0 18px;
display: none;
background-color: white;
overflow: hidden;
}
</style>
</head>
<body>

<h2>Accordion</h2>

<button class="accordion">Section 1</button>
<div class="panel">
<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.</p>
</div>

<button class="accordion">Section 2</button>
<div class="panel">
<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.</p>
</div>

<button class="accordion">Section 3</button>
<div class="panel">
<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.</p>
</div>

<script>
var acc = document.getElementsByClassName("accordion";
var i;

for (i = 0; i < acc.length; i++) {
acc[i].addEventListener("click", function() {
this.classList.toggle("active";
var panel = this.nextElementSibling;
if (panel.style.display === "block" {
panel.style.display = "none";
} else {
panel.style.display = "block";
}
});
}
</script>

</body>
</html>

It will toggle the accordion, once a button/heading is clicked, it will show the content of that section

BabyApple:
Please take a look at this code .. the ".active" class isn't in any other the element but it was declared in the css and JavaScript. How does it work?

<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>
.accordion {
background-color: #eee;
color: #444;
cursor: pointer;
padding: 18px;
width: 100%;
border: none;
text-align: left;
outline: none;
font-size: 15px;
transition: 0.4s;
}

.active, .accordion:hover {
background-color: #ccc;
}

.panel {
padding: 0 18px;
display: none;
background-color: white;
overflow: hidden;
}
</style>
</head>
<body>

<h2>Accordion</h2>

<button class="accordion">Section 1</button>
<div class="panel">
<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.</p>
</div>

<button class="accordion">Section 2</button>
<div class="panel">
<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.</p>
</div>

<button class="accordion">Section 3</button>
<div class="panel">
<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.</p>
</div>

<script>
var acc = document.getElementsByClassName("accordion";
var i;

for (i = 0; i < acc.length; i++) {
acc[i].addEventListener("click", function() {
this.classList.toggle("active";
var panel = this.nextElementSibling;
if (panel.style.display === "block" {
panel.style.display = "none";
} else {
panel.style.display = "block";
}
});
}
</script>

</body>
</html>

The class was attached to the html button by JavaScript. The "toggle" function keyword in JavaScript is what is doing the magic. To see how it is being toggled, if you use chrome.
press ctrl+shift+i to open developer options, now click on the accordion button and watch how the html tag in accordion button is dynamically placed and removed by JavaScript.
A toggle is anything that switch between two options.