They are viewport meta tags, and is most applicable on mobile browsers.
width=device-width
This means, we are telling to the browser “my website adapts to your device width”.
initial-scale
This defines the scale of the website, This parameter sets the initial zoom level, which means 1 CSS pixel is equal to 1 viewport pixel. This parameter help when you're changing orientation, or preventing default zooming. Without this parameter, responsive site won't work.
maximum-scale
Maximum-scale defines the maximum zoom. When you access the website, top priority is maximum-scale=1, and it won’t allow the user to zoom.
minimum-scale
Minimum-scale defines the minimum zoom. This works the same as above, but it defines the minimum scale. This is useful, when maximum-scale is large, and you want to set minimum-scale.
user-scalable
User-scalable assigned to 1.0 means the website is allowing the user to zoom in or zoom out.
But if you assign it to user-scalable=no, it means the website is not allowing the user to zoom in or zoom out.