DFU mode is Device Firmware Update mode - your iPod Touch must be in this mode in order to complete the jailbreak process. It's not something your Mac does, per se, but new Macs running 10.5.6 have issues recognizing this mode in iPhones and iPod Touchs, hence the modification needed to allow it.
A USB hub is
this and using one can avoid the need to make modifications to your Mac's operating system to complete the jailbreaking process.