Imagine you are a painter, and you are drawing a picture onto your canvas. That canvas would be your Godot viewport.
Now imagine that you are drawing another picture onto a separate canvas. This second canvas is completely independent of the first canvas. This would be another viewport.
You could have a third, a fourth, or a fifth canvas (and more if you so chose). At its heart, that's all a viewport is: a canvas upon which you can paint a picture. In computing terms, you can think of it as a window. You can show multiple viewports/windows on the same monitor/screen, or you can show different viewports/windows on different monitors/screens.