Bootstrap is a free and open-source CSS framework. It includes CSS and JavaScript functions for use it in easy way.
Bootstrap CSS Framework is used for build responsive, mobile-friendly web application user interface.
Bootstrap CSS File CDN Path
https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.cssBootstrap JS File CDN Path
https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/js/bootstrap.min.jsExample
<!DOCTYPE html>
<html>
<head>
<!-- Required meta tags -->
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
<!-- Bootstrap CSS Path-->
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css">
<title>Bootstrap Introduction</title>
</head>
<body>
<h1>This is a heading</h1>
<p>This is a paragraph</p>
<!-- JavaScript Files-->
<script src="https://code.jquery.com/jquery-3.3.1.slim.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.7/umd/popper.min.js"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/js/bootstrap.min.js"></script>
</body>
</html>Description
<meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">This is a responsive viewport meta tag. All devices compatable design is created using bootstrap so this viewport meta tag helps in make device friendly user interface design.So most add this to support all device your web application user interface.
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css">This is a main bootstrap css file.
<script src="https://code.jquery.com/jquery-3.3.1.slim.min.js"></script>This is a jQuery file which use for perform some javascript functionality in bootstrap. Javascript functions are provided in bootstrap.min.js file but to use this jQuery js file is required.
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.7/umd/popper.min.js"></script>This is a popper js which provide some additional features in Bootstrap.
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/js/bootstrap.min.js"></script>This is bootstrap js file which have functions for make some event based functionality in Bootstrap.