Complex sentence: Here we make the central field approximation, i.e. that the electrostatic potential is spherically symmetric so is only a function of radius, which is exact for hydrogen, and indeed hydrogen-like systems.
Split into simpler sentences: Here we make the central field approximation, that is, that the electrostatic potential is spherically symmetric so is only a function of radius. This approximation is exact for hydrogen, and indeed hydrogen-like systems.