A simple pendulum consists of a mass, called a bob, attached to a string or rod of fixed length. When the bob is pulled to one side and released, it swings back and forth in a regular motion due to the force of gravity. The time it takes to complete one full swing is called the period, which depends on the length of the string and the acceleration due to gravity. The motion of a simple pendulum can be described as harmonic motion, where the restoring force is proportional to the displacement from the equilibrium position. This concept is fundamental in physics and is often used to illustrate principles of mechanics and oscillations.