It looks just like an ordinary bubble, but green. If it pops near anyone, they get the hiccups and can only get rid of them if one of their hiccups manage to release the bubble.
Role in episode
Timmy and Tina create the bubble and decide to let it loose in Bikini Bottom as a prank. The first victim is SpongeBob, who gets the hiccups because of it, and then passes it on to Mr. Krabs. The bubble keeps getting passed between various characters and eventually returns to SpongeBob, who sadly leaves Bikini Bottom because of it.
Sandy soon finds SpongeBob, and takes him to her treedome to experiment on the bubble.
Eventually, the bubble leaves SpongeBob and ends up in Timmy and Tina's treehouse, and it makes them repeatedly hiccup as comeuppance for creating it.