From Digital Photography School:
One way of adding interest to an image and to draw attention to the main point of interest that you’re attempting to highlight is to incorporate an internal frame into your composition.
A frame serves numerous purposes:
1. It gives the image depth and helps to give the perception to viewers of it that they’re looking at something that is more than 2 dimensions.
2. Use correctly, framing can draw the eye of the viewer of an interest to a particular part of the scene.
3. Framing can bring a sense of organization or containment to an image. This won’t work with every shot – but if you’re after a more ordered or formal feel it can be useful.
4. Framing can add context to a shot. A photo of the woman in the first shot (left) without the window frame would tell you less than the shot you see with the rustic looking window frame around her.
Frames can take many forms – from an overhanging tree, a window, a bridge, arch or even part of another person etc. When using this technique – look for a frame that has a similar shape to the main subject that you’re framing.
Frames can also be in the foreground or background of images (although more often than not they’re in the foreground).