Screening effect is the effect produced by inner core electrons between nucleus and valence electrons. They shield the nucleus from valence electron and effective nuclear charge decreases if there is less shielding effect. Valence electrons are attracted by nucleus and repelled by other electrons.
As Electronic Configuration of Na atom is: 1s² 2s² 2p⁶ 3s¹
So, Inner core electrons between nucleus and valence electrons, that are responsible for screening effect are 1s² 2s² 2p⁶.