To be successful with this question, you will need to know about the two forms of energy. You will also need to understand the concept of mechanical energy conservation.
Energy Forms
Kinetic Energy (KE): Kinetic energy is the energy of motion. If an object is moving , then it has kinetic energy. If an object speeds up between initial and final states, then the final KE is greater than the initial KE. And the opposite is true for a slowing down object.
Gravitational Potential Energy (PE): Gravitational potential energy is the stored energy of vertical position resulting from the interaction of the Earth and an object. If the system includes the Earth and the object, then it possesses this form of energy. The amount depends (in part) on height. Objects at higher positions have more PE than those at lower positions.
Mechanical Energy Conservation
When there is no work being done by non-conservative forces such as friction, normal force, tension force, etc., the total amount of mechanical energy is conserved. The KE is changed into PE and the PE may change back into KE but the sum of the two forms does not change.
In the case of the swinging pendulum bob, we can assume mechanical energy conservation. The tension force acts perpendicular to the bob's motion and does not do work upon the pendulum bob. And we can assume negligible air resistance. So as the bob descends to lower heights, its PE will decrease; its KE will increase to make up for the loss of PE. Similarly, as the bob rises to higher heights, one would observe an increase in PE and a corresponding decrease in KE.
