Q.What is meant by 'shielding of electrons' in an atom?

Step 1. Define shielding. In a multi-electron atom, electrons in inner shells/subshells sit between the nucleus and the outer (valence) electrons, and partly cancel out (screen) the nucleus's positive charge as 'seen' by those outer electrons. As a result, an outer electron does not experience the full, actual nuclear charge Z; it experiences a smaller EFFECTIVE nuclear charge, Zeff, where Zeff = Z - (shielding contributed by inner electrons).

Step 2. Note that different subshells shield with different efficiency. s orbitals (most penetrating, spend more time close to the nucleus) shield most effectively, followed by p, then d, then f orbitals (least penetrating/most diffuse, shield least effectively) -- this ordering (s > p > d > f) is the reason the chapter repeatedly invokes shielding for d and f electrons specifically. …

Defining shielding/screening in general terms, then tracing the specific instances the chapter itself …

• Defining shielding only vaguely ('electrons block other electrons') without the key idea of EFFECTIVE nuclear charge (Zeff) being less than the actual nuclear charge. …