viewport meta tag tells a device to set the viewable area to the width of the device screen. It also sets the initial scale to normal (not zoomed in or out).
<meta name="viewport" content="width=device-width">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
Though forcing initial-scale shouldn't be necessary.
If this is not specified, many smartphones may scale the content down so its content fits within the viewable area, but not any of its padding or margins. This can result in text and images butting right up against the edge of the screen.