A pendulum is a mass (any object, usually metal) on an arm, (a pole or string). The arm is attached to a pivot, which is a point to swing from. The mass will naturally hang down because of gravity, but if it is pushed to one side it will oscillate, which means that it will swing from side to side. The frequency of the swinging pendulum stays the same.
We can work out the time taken for one full oscillation (for example swinging from left to right and back to left. We will call this time T. If the pendulum length is l, and the acceleration of gravity is g, then:
Where π is read 'pi'. Note that the period of a pendulum doesn't depend on the mass of the object.
The regular motion of the pendulum can be used for time keeping, and pendulums are used to regulate pendulum clocks.