Of course it is important that you return the car is a proper state, after all it is someone else's car.
If the car was clean at the beginning of the booking but you returned it dirty, you will need to make sure that you clean it up, wash the car and/or vacuum the inside. Also make sure you remove any wrappers or food remains that were left in the car. If you can't take care of it you will have to compensate the owner so he can take care of themselves, but this is not how it is supposed to go.
If we receive any complaints about the way you've brought back the car. We’ll make sure the car owner gets a clean car. You will have to pay for the car cleaning and we will charge an additional €25 euro's for having to take care of this.