This article was written in 2012. It might or it might not be outdated. And it could be that the layout breaks. If that’s the case please let me know.

Basically

All these different resolutions for all these different devices. It can be pretty confusing and intimidating when you try to understand how they work. Is a pixel a pixel or isn’t it? This can be mindblowingly complex. But here’s what you should basically know:

If you set the meta viewport to width=device-width,initial-scale=1 all devices translate the units you use in your CSS to usable formats on the screen.

That is, if your CSS units are sensible in the first place. This means that a square of 40×40 pixels can be handled with fat fingers on every device. And this means that a 16px (or 1em) font-size is perfectly readable.

The most important part you need to know about the viewport though, is that you should only use it if you optimize your site for all resolutions. In other words: you should only use it if the responsive result is better than the static one.