explain the simple pendulem

Simple Pendulum-  A simple pendulum consists of a small metal ball (called bob) suspended by a massless and inextensibe string from a rigid support, such that the bob is free to swing back and forth.

Thus, we can say that motion of simple pendulum is oscillatory motion.  

When the bob at the rest position is moved to one side and released, the pendulum is set in motion. The rest position is called the mean position of the pendulum. 

In diagram above, Point O represents the mean position of pendulum and Point A and B represent extreme positions. 

Time period - The time period of a simple pendulum is the time taken by the pendulum bob to make one complete oscillation.