Viewport meta tag is a way to tell browsers the width and height of your page's viewport (the visible part of the browser window). This helps browsers render your page correctly, regardless of the device or screen size. Adding a viewport meta tag is essential for pages that are responsive or mobile-friendly, as it ensures that the page will look good on all devices. Without a viewport meta tag, a page may be too wide or too short when viewed on a small screen, which can make it difficult to read or use.