It is a light-brown, live-action meatloaf, with various live-action vegetables around it. It is shown on a white plate, and is, of course, holographic.
Role in episode
Plankton states that it is the only thing he can eat, due to his lack of money, and then proceeds to eat it with a fork. At first, he dislikes the meatloaf because it shows how unsuccessful he is in constrast to his arch-nemesis, Mr. Krabs, as well as how many times he has failed in obtaining the Krabby Patty secret formula. 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 Krabs turn out to be excruciating for him, Plankton returns to his normal life and develops new-found interest for the meatloaf, then proceeding to "eat" it enthusiastically.
Plankton eating a hologram of meatloaf actually makes sense; real life plankton gain energy through photosynthesis, and a hologram is a projection of light.