Q.What is meant by free oscillation?

A free oscillation is what happens when an oscillator is simply displaced from equilibrium and released, with no further outside influence: it vibrates at its own natural frequency (fixed by its own inertia and restoring-force properties, e.g. $\sqrt{k/m}$ or $\sqrt{g/l}$), and in the idealised, frictionless case its amplitude, frequency and total energy all stay constant forever. Examples include a struck tuning fork, a plucked string, or an idealised frictionless pendulum or spring-mass system. In reality, friction and drag inevitably damp a free oscillation's amplitude over time. A maintained oscillation solves this by supplying energy from an external source, exactly once per cycle, in the precise amount needed to replace what damping has removed -- keeping the amplitude constant indefinitely without the frequency being force …