Question

The medium must have inertia so that its
particles may store mechanical energy.
Can anyone please explain this line​

Answer 1

Inertia actually means some kind if resistance to change in state of motion. If a medium has inertia then its particles can resist any change in their motion, i.e.,they'll continue to be in rest as long as an external force changes their state. In fact they have stored energy.

Hope this helps...