The effective nuclear charge is the net charge an electron
experiences in an atom
with multiple electrons.
Higher energy electrons can have other lower energy electrons between the electron and the nucleus, effectively lowering the positive charge experienced by the high energy electron.
A 2s lithium electron can have 2 1s electrons between itself and the lithium nucleus. Measurements indicate the effective nuclear charge experienced by a 2s lithium electron is 0.43 times the charge of the lithium nucleus.