Similar to the Favicon, the apple-touch-icon.png is a file used for a web page icon on the Apple iPhone, iPod Touch, and iPad. When someone bookmarks your web page or adds your web page to their home screen this icon is used. If this file is not found these Apple products will use the screen shot of the web page, which often looks like no more than a white square.
This file should be saved as a .png, have dimensions of 57 x 57, and be stored in your home directory, unless the path is specified in the HTML using the below code.
<link rel=»apple-touch-icon» href=»/custom_icon.png»/>
When this file is used, by default, the Apple product will automatically give the icon rounded edges and a button-like appearance.
Referencia: http://www.computerhope.com/jargon/a/appletou.htm