Below is a problem which states a fact about "Tchebyshef net". I don't understand meaning of bolded part.
The coordinate curves of a parametrization $x(u, v)$ constitute a Tchebyshef net if the lengths of the opposite sides of any quadrilateral formed by them are equal. Show that a necessary and sufficient condition for this is $$\frac{\partial E}{\partial v} =\frac{\partial G}{\partial u}=0.$$ Reference: Differential Geometry of Curves and Surfaces [Manfredo P.do carmo] Page 100 Problem 7.