A swing at the park is an example of a pendulum. The period of a swing, or pendulum, is the time required for the pendulum to circle back to its beginning point, after traveling outward (i.e., from A to B back to A).
Conservation of energy maintains the motion of the pendulum, as Gravitational Potential Energy (GPE) is converted to Kinetic Energy (KE), and KE is converted back into GPE. A swing at the park is just a pendulum with a length, and you are the mass. As you swing, the angle with respect to vertical changes.