The total derivative of a function assigns for every point of the domain a linear map with same domain, which is the best linear approximation to the function value around this point, i.e. the tangent plane.
E.g. in 1D:
Totalderivative=D[f(x0)](x)=f(x0)+∂x∂f(x0)×x
and in 2D:
D[f(x0,y0)](x,y)=f(x0,y0)+∂x∂f(x0,y0)×x+∂y∂f(x0,y0)×y
