What is actually a resonating vibration and resonance?
I have searched many books and made Google search too but couldn't understand it clearly.
Consider a pendulum, such as a playground swing.
It is a 2nd-order system, because it swings in a repetitive motion at a certain frequency like, say, 30 swings per minute. It also is damped, because if you set it swinging and then leave it alone, it rubs against the air and its swings becomes smaller and smaller until it seems to have stopped.
Now, if you give a shove (put energy into it) at the same frequency that it swings (30 times per minute), the swings will get larger and larger until they get really large. That's what happens when you drive a 2nd-order system at its resonant frequency.