Question

explain the different positions of the bob of the simple pendulum while it oscillation

Answer 1

The answer is very simple, it's Law of conservation of energy.

First of all, any oscilatory motion starts if the object under study is disturbed from its equilibrium position (mean position).

In this, the external agent disturbs the equilibrium by providing additional potential energy and when released, due to restoring force/torque, the oscillating body tries to achieve mean position but when it does so, it attains kinetic energy (hence momentum) so it does not stop their and goes ahead.