When the pendulum swings, the potential energy is converted to kinetic energy and vice versa.
Potential energy is related to the height of the object. The higher it goes, the more potential energy it has. You can see it from the formula PE = mgh.
Kinetic energy is related to the speed of the object. The faster it is, the more kinetic energy it has. The formula is KE =
When you raise a pendulum, it gains PE. While it drops altitude, PE decreases. You can also see that the pendulum drops faster and faster. This is because the PE lost is converted to KE. When it hits the lowest position, all the PE becomes KE, which means that the speed of the pendulum is the highest at this moment. Then, the pendulum goes up again and gains PE while it loses KE, until it reaches the highest position. Then, it repeats all over again.