A beautiful view, as breathtaking as it may be, cannot give us all the emotions it has to offer if we look at it from a very small window that is facing the wrong way. Sometimes a wider window is what we need. A similar concept can be applied to the digital world with viewports.
A viewport is generally the size of a visible portion of User Interface in a screen. Devices with different screens are characterised by different viewports. The viewport, usually expressed in pixels, can be very big, as in a full HD screen, or very small such as on a smartwatch. Those who design software use viewports as a reference to decide how to vary the layout of the elements so as to optimise the User Experience.
Referring more specifically to web development, viewports identify the set of rules which tell web browsers what to show on the screen, based on their size. A developer configures the viewports to allow users to consult the pages easily regardless of the device.