What is Orbital energy and radius?

In Slater’s model, the orbital energy of an electron of effective charge $Z^*$ and effective principal quantum number $n^*$ is $\varepsilon = -E_RZ^{*2}/n^{*2}$, and its orbital radius is $n^{*2}a_0/Z^*$, the most probable radius of a hydrogen-like orbital of charge $Z^*$.