The Nyoibō is a staff or rod-like weapon that elongates and shrinks at the whim of it's master. It's purpose is to connect Karin's Palace to Kami-Sama's Palace, and it's how Son Goku first reaches the place. However before that, it's used primarily as a weapon and it's origins are never questioned nor explained.
Son Goku inherited the Nyoibō from Son Gohan Sr.. After using it to reach Kami-Sama, he never seems to use it in battle again. It's unknown if it was inherited by either of his sons or his protege, but the Rod weapon-class in Dragon Ball Online is based on the Nyoibō heavily.