What is the poor shielding effect?

Hint: The shielding effect is the decrease in the attraction between an electron and the nucleus in an atom. The shielding effect depends upon the electron density of the orbitals. S have the most shielding effect and f orbitals have the least or negligible shielding effect.

Complete answer:
Shielding effect: suppose there is an atom that contains many electrons in its orbitals. The electrons present in the inner shells are closer to the nucleus and the electrons on the outer orbitals are far from the nucleus. So the electrons present in the inner orbitals will experience more nuclear charge than the outer orbitals electrons. The outermost shell does not experience the complete nuclear charge because of the repulsive interaction of the intervening electrons. Thus the outermost electrons are shielded or screened from the nucleus y the inner electrons this is called the shielding effect or screening effect.
Now, when these inner shell electrons cannot effectively shield the outermost electrons from experiencing the effective nuclear charge, it is known as the poor shielding effect. The s and p orbitals are considered to be the most effective in shielding and f and d orbitals are least effective in shielding. This difference is because of the electron’s density. S and p have a higher density of electrons so they can shield more effectively than f and d orbitals.

Note:
Poor shielding effect results in inert pair effect. Resistance of $n{s^2}$ electrons to take part in bonding due to the poor shielding effect of intervening d and f orbitals is called the inert pair effect. It can be said that f orbitals have a negligible shielding effect because of lanthanide contraction.