It is a golden-bronze ring with miniature plastic versions of SpongeBob and Patrick inside it. The song they sing goes like this:
It's the best friends forever,
Best friends forever, ring!
Role in series
"The Secret Box"
Patrick tells SpongeBob that if he tries to peak into his secret box one more time, they will no longer be friends, in regret of joining the Best Friends Forever Club. The camera then zooms in on the friendship ring on Patrick's finger.
Later at night, when SpongeBob breaks into Patrick's house, Patrick suspects that the clam burglar has broken into his house, but it actually turns out to be SpongeBob, as he reveals his true identity by showing him the friendship ring on his finger, which then proceeds to play the song.
When SpongeBob is singing "Sweet Victory," he pulls out his hand and the friendship ring is briefly shown.
SpongeBob's Truth or Square
It is one of SpongeBob's memories.
SpongeBob's Game Frenzy
The ring appears in the Best Friends Ring game. In order to win, the player has to turn the crank a certain number of times. If they do so, the game is won. However, a loss causes the ring to malfunction and sing the opposite lyric: "It's the worst friends forever ring!" Also the victory line is shortened to "It's the best friends forever ring!"
Coincidentally, the two episodes that the friendship ring appears in are sister episodes.