What is Screening, effective nuclear charge?

In an atom with several electrons, an electron is attracted by the nucleus, of charge $Ze$, and repelled by the other electrons. The inner electrons partly cancel the attraction of the nucleus: this is screening. The valence electron behaves as if it felt a reduced nuclear charge $Z^{*}e$, the effective nuclear charge, with $Z^{*} < Z$. Electrons of an inner shell screen almost completely; electrons of the same shell screen only a little.