When an object vibrates, it sets the particles of the medium around it vibrating. The particles in the medium in contact with the vibrating object displace from its equilibrium position. It then exerts force on the adjacent particles. After displacing the adjacent particle the first particle of medium comes back in its original position. This process continues in the medium till the sound reaches your ear.
When the bell continues to move forward and backward, it creates a series of compressions and rarefactions making production of sound.
Sound waves needs material medium to propagate therefore, they are called mechnical waves. Sound waves propagate through a medium because of theinteraction of the particles present in that medium.
Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend?
No, because sound waves needs a medium through which they can propagate. Since there is no material medium on the moon due to absence of atmosphere, you cannot hear any sound on the moon.