The quotation you allude to is found in "Letter 1," signed "Your affectionate uncle, SCREWTAPE":
Remember, he is not, like you, a pure spirit. Never having been a human (oh, that abominable advantage of the Enemy's!) you don't realise how enslaved they are to the pressure of the ordinary.
The meaning of this is that being earthbound, humans are bound to do that which is ordinary. He contrasts that to what a "pure spirit" can do. Examples of the ordinary are farming, eating, raising children. Such things are the ordinary elements of an earthbound, non-spiritualized, life. Screwtape gives the example of having lunch in the next part of Letter 1:
I struck instantly at the part of the man which I had best under my control, and suggested that it was just about time he had some lunch.