Depth of Field in Poser enables you to focus on select areas in your image by choosing a focal point. Objects close to this focal point will be sharper while those further away will be out of focus or "blurry".
Depth of Field is especially useful in large scenes as you can direct the viewer's eye to your subject. It also makes for more powerful portraits by highlighting and emphasizing selected details. For example focusing on the eyes makes the portrait stand out more.