Question

what is a simple pendulum ?how is it used for measuring time answer in short

Answer 1

A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced sideways from its resting, equilibrium position, it is subject to a restoring force due to gravity .

Answer 2

fixed-mass pendulum in a fixed gravity field, the period is constant. This means the device can be used to measure elapsed time. Just count the swings during the event you wish to measure, and you have the elapsed time accurate to one swing.