Step-by-step explanation:
Remember how the pendulum swings back and forth. Along the path of the swing the pendulum mass gains height before swinging back down to the bottom and then back up again.
This motion is exchanging potential energy (stored) for kinetic energy (in motion).
Remember that for motion, potential energy is proportional to the height of an object.
where z is the height, m is the mass, and g is the acceleration of gravity.
So when the pendulum mass is at its lowest point, it has converted all of the potential energy into kinetic energy.
The why is because it has fallen as far as the string will let it along its path of motion - thereby converting all the potential energy to kinetic energy.