I need some help to understand a claim in a paper that I'm reading.

Let $v, z: I\subset \mathbb{R}\to \mathbb{R}$ solutions to the ODE $$\left\{ \begin{array}[cl]. v' &= h(z)\\ z' &= h(z) \tan v \end{array}\right.$$ Where the function $h: \mathbb{R} \to \mathbb{R}$ is a diffeomorphism and this is the only information about $h$.

I already know there exists these solutions. The paper says "$e^z\cos(v) $ is constant along the solutions of this ODE". What can mean "constant along a solution" in this case? And, how can this information help me to understand a solution of this ODE?