It is a tear drop that is made out of the shape of the heart.
Role in movie
When SpongeBob and Patrick are getting heated under a lamp and almost become inanimate, they both cry out one tear of joy which joins together to make one tear shaped like a heart. When SpongeBob and Patrick become inanimate, the tear slides down the lamp's wire into the electrical outlet, which short-circuits and lets out smoke. The smoke rises to the ceiling and sets off the fire sprinklers, which causes all the dried-up sea creatures in Shell City to come back to life, along with SpongeBob and Patrick.