What is activating effect? In the book "Organic Chemistry" by Morrison and Boyd,It is written that


  Sodium borohydride cannot reduce weaker electrophiles like esters though Lithium aluminum  hydride can reduce esters to alcohols due to stronger activating effect of Li+ ion than Na+


What is activating effect?

(Google search did not give appropriate results.)
Consider a carbonyl group. We know that, under acidic conditions, $\ce{H+}$ can protonate on carbonyl $\ce{O}$ and hence activate the carbonyl group to nucleophilic attack at $\ce{C}$, and we call such an effect an activating effect. The metal ions $\ce{Li^+}$ and $\ce{Na^+}$ serve the same purpose as $\ce{H+}$, though $\ce{Li^+}$ is a stronger activator due to its high charge density and hence stronger attraction to $\ce{O}$.