It is presumably two or three scoops of vanilla ice cream with chocolate sauce "hair" and a banana and cherry hat. It has banana and red cherry arms and three chocolate (or other candy) buttons for eyes and a nose with a big red licorice smile.
SpongeBob and Patrick eat them very quickly and rapidly, which causes them to get a big ice-cream headache the following day. All the mess that SpongeBob and Patrick make flies onto the Goofy Goober Waiter.
It is mentioned in the episode "The Clash of Triton" by SpongeBob; however, he pronounces it as "Gooberry," not "Gooberberry." Harold "Bill" Reginald also asks for one at the ice cream shop in "Porous Pockets." However, he asks for extra nuts, and there are zero on the sundae.