The Viewport is probably the most important aspect of Responsive Web Design. I was surprised that I had some code from 2011 that had a Viewport Meta Tag. I admit that I had no clue what it was at the time.
As usual I took my main inspiration from the W3C website and lookig at my CSS for responsive design I must have copied a whole chunk of code from there not really knowing what I was doing. The Clearfix is a prize example of this as born out by the .row class that clears the floats for the layout. The MDN pages give a fuller description of the Viewpoint tag and also point to the CSSWG (CSS Working Group) that seems to be the source document.
W3C explain: (This is a simplistic overview)
The viewport is the user's visible area of a web page.
For a fuller explanation go to the W3C website.
The responsive nature of a website and pages on that website is key to whether a page is usable on all devices.