The frame within a frame trick, sometimes known as foreground framing, is a great way to lead the viewers eye through to the main focal point of an image, and to keep it there. The general idea is that the main subject is inside a real or implied frame, and usually other parts of the image are blocked out.

There can be several benefits in using this type of composition, for example it can provide a sense of context, that is, tell you something more about the subject, as in this wedding photo of a groom as the bride and her dad walk down the church aisle towards him:-

wedding procession
It can also provide the image with a sense of depth and layers as in this image:-
Even though your subject might only reflect a small proportion of the frame, using a window or doorway to provide a clean surround can really add impact.
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You often see the frame within a frame trick being used in films and TV dramas to show that the subject is being spied on and completely unaware of being watched (they're usually murdered seconds later!)
One terrific way of using this 'being spied on' technique is taking photos through a doorway of say children playing, showing them in a natural rather than a posed way .