As valence electrons are farther from the nucleus than the core electrons, the core electrons "shield" the valence electrons from the full nuclear charge, or push the electrons farther from the nucleus so that they experience a smaller nuclear charge.
For example, Na's valence electrons are shielded by the 1s, 2s, and 2p electrons, therefore they experience a +1 charge while the core electrons experience the full +11 charge of Na's nucleus
s electrons shield the best, followed by p, then d, then f
