It is simply a Krabby Patty in a much smaller size.
Role in episode
After Junior starts nibbling on Patrick, he and SpongeBob feed him various foods to satisfy his hunger. SpongeBob offers him a regular Krabby Patty, which a disgusted Junior turns down. Patrick singles him out and states that food like that is not a good choice for someone as small as Junior, so SpongeBob offers him a tiny Krabby Patty, but Junior still doesn't like it and would rather eat the Worm from Appleworld, which SpongeBob and Patrick end up feeding him.