Is unlimited Internet really unlimited? What is tethering because I know this isnt allowed but I don't even know what it is?
Giffgaff state that the internet is unlimited but will bar you if you use more than 1GB in an hour. In theory they'll then reinstate it if you confirm that you'll abide by the T&C, but some users report having been permanently banned.
Don't worry about tethering; you've got an iPhone and you'd need to jailbreak and then install 3rd-party apps to tether. So long as you don't do that, you won't end up tethering!