The function tells whether or not the current page being displayed is being viewed by a mobile device. It is helpful if you want to hide certain parts of your site such as graphics, when being viewed on a phone.
[if ! mobile]img:graphic.jpg[if]
Note: Technically this conditional checks 2 things. If it is a mobile device but you are not using the mobile skin for some reason, it will return true. Similarly, if you are viewing it on a desktop computer, but you have set the skin to mobile, it will also return true.