The world is full of objects, which make perfect natural frames, such as trees, archways, and holes. There also many human made objects that can be used to frame the subject of the photograph.
By placing these around the edge of the composition the photographer is able to isolate the main subject from the rest of what is in the image. The result is a more focused image, which draws the eye of the viewer to the main point of interest.
You can have a frame at the front or behind your subject.