A tethered cable refers to a charging cable that is permanently attached to a charge point, it cannot be removed and taken away.
An 'untethered' cable is one that you can unplug from the charge point and taken away. An example of a public charge point that would need you to bring your own cable would be an on-street lamp post charger.
It is essential to keep a charging cable in your car so that you are able to charge from untethered points if required.