It can be used as a literary device. For example a motif can be seen in image, sound, action or other figures that have a symbolic significance and contributes toward the development of the theme. Motif itself explains the theme of the story.
Example:To Kill a Mocking Bird= Lee strengthens the general scene by a motif of gloomy and haunted settings, supernatural events, full moon, etc. Another motif is the small town life of Maycomb, which shows goodness and pleasantness in life.
However there are more definitions of motif. I suggest you go to this website: Dictionary.com and search for motif.