I've a fairly strong web developer background where a viewport is basically the visible area of a webpage for a user. So for example, a page displayed on an iphone in landscape would provide a completely different area to play with than a 28" mac pro in safari.
I haven't used them in my construct 2 projects yet but I would imagine it is the same.
Basically it is a way of making sure people iwth different size screens all see the same thing, or atleast what you want them to
Hope this helps