The <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no"> line is an HTML head meta tag that ensures your web page appears correctly and compatibly on mobile devices. It is a cornerstone of responsive design.

Explanation:

<meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">

Parameters:

width=device-width: Sets the screen width to the actual pixel width of the device.
initial-scale=1: Displays the page at 100% zoom level (zoom 1) when it is first loaded.
shrink-to-fit=no: Prevents the content from being shrunk to fit the screen size. Commonly used in Safari.

Why is it Necessary?

Prevents display distortions on mobile devices.
Responsive CSS frameworks (e.g., Bootstrap) require this tag.
Since browsers' default width is usually 980px, mobile-compatible CSS will not work without this tag.

Example Usage with Mobile-Friendly CSS:

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
  <link rel="stylesheet" href="style.css">
</head>

body {
  font-family: sans-serif;
  margin: 0;
  padding: 0;
}

.container {
  width: 100%;
  max-width: 1200px;
  margin: 0 auto;
  padding: 15px;
}

@media (max-width: 768px) {
  .container {
    padding: 10px;
  }
  .menu {
    flex-direction: column;
  }
}

Summary:

Without this meta tag, responsive CSS codes will not work effectively. It must be used between the <head> tags to ensure that your page is rendered correctly on mobile devices. It should be included by default, especially in modern frontend development processes.