A thumb is a digit on a human hand, and is sometimes considered a finger. It is the hand's most prominent digit.
Raising one or both thumbs while keeping the other fingers clasped to the hand is known as a "thumbs up" and is generally considered a positive gesture. It can communicate several messages, including "I'm fine", "I approve", and "I am feeling impressed with myself. Fonzie is fond of this gesture. However, a similar gesture, where the thumb is pointing down, is called a "thumbs down" and communicates disapproval.
When Mork froze Fonzie with the abilities Orkans have, Fonzie's thumb was the first part of him to unfreeze.