When you listen to your friend speaking, what is going on to allow you to hear their words?
What's happening is when your friend speaks, their vocal cords vibrate, and these vibrations are occuring at a molecular level. The vibrating vocal cords cause the atoms nearby to vibrate and a chain reaction is created. These atoms in turn bump into nearby atoms and cause those to vibrate, and this continues on and the chain reaction progresses away from the source. Eventually the vibrations reach your ear, and the complex bone and sensory systems in your ears convert this mechanical energy into chemical and electrical signals that the brain interprets and so the mechanical energy created by the vocal cords of your friend is transferred to your ear through the movement of particles in the air.
Specifically, sound is our perception of a medium being set into motion and sound is a pressure disturbance that travels through a medium as a mechanical wave. When we create sound, by typing on a keyboard or gasping at a shocking scene in a movie, we create vibrations that cause the air molecules around us to move. These vibrations are spread out from us, causing pressure waves as the molecules transfer energy by bumping into adjacent molecules.
Pressure waves consist of oscillating areas of high and low pressure (i.e. areas with more/fewer particles) spreading away from the source. Areas of high pressure are known as compressions and areas with low pressure are called rarefactions.
A medium is a substance through which energy may travel. In this case, the medium may be any solid, liquid, or gas. A medium is the material that carries the wave.
Consider new and media (plural). You've probably heard of news networks and journalists being described as the media. The media refers to newspapers, television networks, radio stations, and other outlets. The news travels through the media and the media are the things carrying the news from the source to your television set and ultimately to you.
Another way to think about it is with how the ocean moves. The waves on the ocean travel through the water. So the water is the medium.
Try this: find a tuning fork or borrow some from your music friends. Hold the bottom of the tuning fork (the ball end), lightly whack your kneecap with it, and bring the tines up to your ear. Hear that sound? How is it happening? We know sound is essentially because of vibrations but you likely cannot see the fork vibrating. Now grab a cup of water. Then whack the fork again, only this time, slowly dip it into the water after. You will see that the water moves and creates ripples and waves, as if someone dropped something in the cup. Actually, if the fork is big enough and you hit it hard enough, a considerable mess may be made. This is an easy way to see the vibrations of the tuning fork! Check out the video below.