A pendulum is a weight suspended from a pivot, when you give a force, it can swing freely.
The picture below shows a pendulum and it's variable. To "Simple gravity pendulum" it means no friction or air resistance.
Period: T2=4PI2*L/g
where, g = Acceleration of Gravity = 9.8, L = Length, T = Period.
For example, when Length = 1, g = 9.8, The T = 2.84S.