width: By default, mobile browsers pretend they have a width of 980px when laying out a web page or even when evaluating media queries. Setting the width to device-width tells the mobile browser that you are prepared for small screens, and they should do everything with their true width.
initial-scale: 1 means that one CSS pixel equals one viewport pixel. (On retina screens one viewport pixel equals four device pixels.)
maximum-scale=n: The max zoom allowed. Better to leave users in control.
user-scalable=true|false: Forbid zooming in/out. Probably better to leave this out!