Water roux, commonly called tangzhong, is a cooked paste of flour and liquid added to bread dough to help it retain moisture and stay soft. A portion of the recipe's flour is stirred with water, milk or both over heat until it thickens, then cooled before mixing. Account for that flour and liquid in the total formula.
It differs from yudane, in which boiling water is poured onto flour to make a scald. Neither paste is the fat-and-flour roux used for sauces. Follow the bread formula rather than substituting these preparations by volume.
Reference: Tangzhong and yudane.