It is a white and gray piece of briefs with some red rectangles on its top.
Role in series
SpongeBob and Patrick are both in a grapple, each straining to bring the other one down, until the strain finally proves too much for their pants to withstand. Both of their pants began to rip and fall down revealing their briefs. Patrick's underwear is yellow and SpongeBob's underwear is pink—the color of the other person—leading to them both crying and hugging each other. Moved to tears, the two friends reconcile and agree never to fight each other again, walking out of the arena.
When Patrick says he doesn't want to die in his underwear while they are being tormented by Squidward (who is dressed up as The Fisherman, SpongeBob offers him his own underwear. Patrick accepts the offer, and he puts on SpongeBob's underwear, while SpongeBob puts on Patrick's.
Role in video games
When SpongeBob is hit by an enemy and wear his pants, he will lose his pants, showing his underwear. SpongeBob's clothes work as a life points counter. This means if he's hit with his underwear he will be naked and lose. So be careful.
Once again, his underwear works as a life points counter. SpongeBob starts with three pairs of underwear, but if the player finds three pairs of golden underwear, he will have six pairs of underwear. Each pair of golden underwear gives the player capacity to wear one more pair of underwear. The golden ones don't respawn. If the counter is empty, the player will return to the previous checkpoint.