This property allows you to change the URL of the node without changing the name of the node/page you have created. Create it as a textstring property.
For example, in the Gallery we have a gallery called the bookhouse boys with a photo called Catherine Turner and Paul Van Oestren which creates the following URL by default
http://localhost/gallery/the-bookhouse-boys/catherine-turner-and-paul-van-oestren.aspx
If we were to enter catherine-turner in the umbracoUrlName property you will change the URL of the page to
http://localhost/gallery/the-bookhouse-boys/catherine-turner.aspx