A pickle is a cucumber that has been pickled in a brine, vinegar, or other solution and left to ferment for a period of time, by either immersing the cucumbers in an acidic solution or through souring by lacto-fermentation.
Role in series
Pickles are the driving point of this episode. SpongeBob is reminded by Bubble Bass that he forgot to put pickles on his Double Triple Bossy Deluxe, which destroys his memory of how to make a Krabby Patty, let alone do anything.
Toward the end of the episode, it is revealed by SpongeBob that Bubble Bass had been hiding the pickles under his tongue the whole time, among many other belongings of the Bikini Bottomites.