When is it permissible to rub hands on something instead of washing them?
When there isn’t any water available after using the bathroom, or touching a covered part of one’s body etc., one can rub his hands on something. The reason is because we are washing our hands for cleanliness. However before eating a bread meal, we have to walk 18 minutes out of our way (or 72 minutes if it not out of the way) if there is water available there, and if there isn’t any water available we cover our hands and eat, but wiping them doesn’t help. The reason we need to specifically wash them is because we are washing them because of tuma and tehara.
O:CH 4-22, 163-1.