A tethered charger is one where the lead you plug into your car is attached to the wallbox supplying the electricity – so no need to take the charging leads out of your boot. Most of these come with a Type 2 plug, which is fine for the majority of modern EVs, but some cars have a different type of socket, so the plug won’t work.
Untethered chargers have a detachable cable, they are often smaller and cheaper and are also considered tidier.