A salad is a food consisting only the Krabby Patty's tomato and lettuce that was sold at the Kuddly Krab in "Bossy Boots." Before it was sold there, Mr. Krabs opened a salad bar as a gimmick to get customers in the episode "Culture Shock."
In "Bossy Boots," SpongeBob was supposed to make two salads. Pearl Krabs made one by removing everything from two Krabby Patties except the tomato and lettuce. SpongeBob was shocked and apparently had never seen a salad before this day. Even though he was broken-hearted after destroying a couple of Krabby Patties, he sadly brought them to the customers. They appeared to be sad too.
In "Culture Shock," the Krusty Krab appeared to have a salad bar. In this particular episode, Mr. Krabs made the salad bar free after having a very long loss of customers. The salad bar is also seen in "My Pretty Seahorse" when Mystery eats out of it. Ironically a side salad is usually seen on the menu, indicating that SpongeBob does know how to make a salad. Mystery also ate Mr. Krabs' money thinking it was salad.
SpongeBob calls the salads by "Sa-lads" as a running gag in the episode "Bossy Boots."
In "Bossy Boots," SpongeBob doesn't know what a salad is.