Tethered EV point means it has a type 2 cable attached to the unit which you plug into your car, whilst untethered comes with a socket that you plug your own cable into.
Tethered units take the fuss away from having to get a cable out of your vehicle for easy and quick charging. There is also no risk of misplacing or forgetting the charger lead.
Tethered leads are approximately 6.5m long but there are a variety of lengths from each manufacturer.
Untethered units are for the most part look neater as there are no cables to tidy around the unit. There is also the option to upgrade to cable in the future. Tethered chargers are best when used frequently to save time, and save damaging a portable lead.