Separation of time scales means that the characteristic times of movement (oscillation period, damping time, etc.) of the two systems are very different; this is also known as adiabatic approximation. If we take for simplicity that both nuclei and electrons perform oscillatory motion with periods $$T_n$$ and $$T_e$$, then $$T_e \ll T_n,$$ so that we can think of nuclei as static when solving the equations-of-motion for the electrons.