I am taking v to be a continuos and differentiable function of x$$v=f(x)$$
Then
$$\frac {dv}{dx} = 0 $$ means either v is maximum or minimum at this value of x. This is following simple Calculus.
Without any additional information about its function, it is difficult to say anything else.
If the motion is in one direction only (moving forward only), then it means maximum.
But if it follows a complex trajectory, then it should perhaps represent both maximum and minimum.
You can further check whether it is maximum or minimum with simple Calculus as follows :
For maximum,$$\frac {d^2v}{dx^2} < 0$$
For minimum,$$\frac {d^2v}{dx^2} > 0$$
In case $$\frac {d^2v}{dx^2} = 0,$$ then it is neither minimum nor maximum.