Just as shutters frame a window, and just as your lashes and eyebrows frame your eyes, proper framing of your subject draws and concentrates attention on it, making it more interesting to the viewer.
A lot of things can serve as a frame for your subject.
In many cases, having your frame darker than your subject is quite effective, but at any rate your frame should be kept simple so that it doesn’t distract from your actual subject. The frame can also help reinforce the mood of the picture, and should not conflict in style with the message you want to convey with your image.
For more information on this topic, you can also listen to the following TFTTF show: