"The shielding effect is defined as a reduction in the effective nuclear charge on the electron cloud, due to a difference in the attractive forces on the electrons in atom"
The Shielding effect increases down the group in periodic table because shell number increases - shell number increases mean electron increases - as the result inner electrons will shield the outer electrons so outer electrons will experience that less nuclear charge than actual
The Shielding effect remain same in the periods - because shell number remains same along the period.
Electrons present in the inner shells Shield the force of attraction of nucleus felt by the valence shell electrons is called Shielding effect
For example if we compare the shielding effect of sodium and potassium
