What is the full sentence? There is a saying: "Her kitchen is so clean that you can eat off the floor" This means that the woman has an extremely high standard of cleanliness in her house and her kitchen floor is so clean that you could eat food off the floor. It is not meant literally as few people would do this.

It means: Don't eat any food that has been on the floor, dropped or otherwise.

it means licking off the floor is ok, but not eating :P