in the case of a single-celled organism, no specific organs for taking in food, exchange of gases or removal of wastes may be needed because the entire surface of the organism is in contact with the environment. What is meant by this?

This means that single-celled organisms such as bacteria have semi-permeable membrane surrounding them through which movement of nutrients, wastes, gases etc takes place. So, single-celled organisms take up nutrients, exchange gases, eliminate wastes etc. by diffusion through the cell membrane.