Question

Explain how the gain of electrons takes place in the process called "REDUCTION"?

Answer 1

Reduction is process of addition of hydrogen/electropositive element or removal of oxygen/electronegative element.

As hydrogen is electropositive element, it try to lose electron, and the electron density of the element connected to will increase.
Oxygen is electron negative (tend to pull electron pair toward itself), and removal of oxygen will again increase the electron density of the element which was connected to it.
Thus in reduction process electron density increases.
O₂ + 2H₂ = 2H₂O, here oxygen is reduced.