Jantou「雀頭」 means the pair in the winning hand. In English, it can be referred to as "the pair" or the eye. Every winning hand requires one jantou. Even the irregular yakuman kokushi musou requires one jantou. Chiitoitsu is a notable yaku, because it requires seven of them.
Certain tile pairs produces fu. Each produces 2 fu. Any other pair combinations not listed is worth 0 fu.