It is a light brown, live-action form of meatloaf with various live-action vegetables around it. It is, evidently by the name, holographic.
Role in episode
Plankton states that this food is the only thing he can eats and then proceeds to eat it with a fork. At first, he dislikes the meatloaf because it shows how unsuccessful he is in contrast to his nemesis, Mr. Krabs. As a result, he slams the food off the table in anger and refuses to eat it.
Later on in the episode, after the events of switching lives with Mr. Krabs turn out to be excruciating for him, Plankton returns to his normal life and develops a new-found interest for the meatloaf, then proceeding to "eat" it enthusiastically.
Role in Which SpongeBob Character Are You? and Which SpongeBob Side Character Are You?
Holographic meatloaf appears as one of the selectable choices in both of the games.
Plankton eating a hologram of meatloaf actually makes sense; real-life plankton gain energy through photosynthesis, and a hologram is a projection of light.