klein-gordon equation Definition and 1 Threads

I am reading Relativistic Quantum Mechanics Wave Equations, 3rd ed. - W. Greiner and I'm on section 1.3 looking at a solution ##\psi## to the Klein-Gordon equation in the nonrelativistic limit. The solution is split up: $$\psi=\phi\exp{\left[-\frac{i}{\hbar}m_0c^2t\right]}$$and we then show...