It is a white glass plate with a tiny black dirt spot on it, which appears to be difficult to remove.
Role in episode
After SpongeBob finishes cleaning the dishes and is about to leave work, Mr. Krabs points out a plate with a missed spot and commands him to clean it off completely before heading out the door.
The spot is tough to remove. SpongeBob pulls off many bizarre attempts to clean it off, such as hitting it with a baseball bat and chainsaw, dropping a kitchen sponge on it, and using his saliva, but to no avail. After many failed attempts, SpongeBob utilizes his new device, the Spot-Master 6000.
The spot is finally removed, but destroys the Krusty Krab in the process.