In photography, depth of field (DOF) refers to the distance between the nearest and furthest objects in an image that appear acceptably sharp.  In addition to the distance between the subject and the camera lens, various properties and settings of the lens (e.g., focal length, f-number, and format size) affect the DOF that can be achieved.  A shallow DOF tends to emphasize the subject of interest while de-emphasizing the foreground and background.  In traditional photography, achieving a shallow DOF typically requires more expensive equipment. 
Some existing image processing applications offer a post-processing feature to mimic the effects of a shallow DOF.  After a picture is taken and stored to disk, the user selects an area in the image to emphasize (e.g., by drawing a boundary box using the mouse), and the software applies blurring effects to the rest of the image.  Such techniques, however, tend to be computationally intensive, and are typically unsuitable for use on mobile devices such as smartphones.