At the highest points of its swing the pendulum is still; its kinetic energy is zero and its potential  energy at its greatest.
As the pendulum swings down and gathers speed, its gravitational potential energy is being converted to kinetic energy.
At the bottom of its swing the pendulum is moving its fastest; its kinetic energy is at its highest and potential energy its lowest.
As the pendulum climbs the other side of its swing it slows down; its kinetic energy being converted back to potential energy.
Without any friction the pendulum will keep swinging forever, its total energy constant, but continuously exchanging between kinetic and potential energy.
Conservation of Energy (and Mass)