What is damping in Simple Harmonic Motion?

An oscillation is damped if resistive forces are present e.g. air resistance or friction.
The amplitude of the system will decrease over time, as opposed to a free oscillation which is undamped (no resistive forces) and will have a constant amplitude.
Light damping occurs when the resistive forces acting are small – many oscillations occur but the time period stays constant as the amplitude falls. E.g. simple pendulum in air.
Critical damping occurs when the system stops oscillating after the shortest possible time. E.g. A car suspension system
Heavy damping occurs when the resistive forces acting are large – not even one complete oscillation occurs as the system slowly returns to equilibrium. E.g. A push tap in a public toilet.