I saw this when backing up my iPhone, and I have no idea what it means. It is in the space where the X/cancel symbol usually is, and it had no tooltip when I hovered over it.
This symbol appears during the process of syncing between an iPhone, iPad or iPod with iTunes on a Mac or PC. I can't recall the official name of this iTunes symbol, but I can tell you what it does.
While iTunes is performing a sync it works through a number of steps. This symbol appears when the "Backup up" step commences (usually Step 2). By clicking on this symbol iTunes will skip this step and move on to the next step. In a nutshell it is a skip backup symbol, although I can't recall if that's the offical name.
This is a great option if you just want to quickly sync some purchases etc without waiting for iTunes to first do the backup, especially if you've recently done a backup or are in a real hurry.