It means the iMac is only seeing the iPad as a USB (camera) device, and not as an iOS device.
I'm not an expert on this problem, but here are some of the things I've heard suggested.
Try another USB port.
Make sure you are running the current version of iTunes.
Reset the iPad: Hold down the power and home buttons until the Apple logo appears. The iPad will reboot. This usually takes a bit longer than a restart.
That is all I can remember.
If no one wanders by with the answer, try searching the forum for the problem, or go to apple.com/support and search there.