The viewport is the user’s visible area of a web page.
The viewport varies with the device and will be smaller on a mobile phone or iPad than on a computer screen.
Before tablets and mobile phones, web pages were designed only for computer screens, and it was common for web pages to have a static design and a fixed size.
Now, we started surfing the internet using tablets and mobile phones & iPad, fixed size of the web pages was too large to fit the viewport.
HTML5 introduced a method to let web designers take control over the viewport, through the <meta> tag.
By including this <meta> tag, the viewport element will give the browser instructions on how to control the page’s dimensions and scaling.
Ex:
<meta name=”viewport” content=”width=device-width, initial-scale=1.0″>