What Does It Mean If Directional Derivative Is 0?

What Does It Mean If Directional Derivative Is 0? The directional derivative is a number that measures increase or decrease if you consider points in the direction given by →v. Therefore if ∇f(x,y)⋅→v=0 then nothing happens. The function does not increase (nor decrease) when you consider points in the direction of →v. Can directional derivative