To start understanding what sound is, take a look at the red dots below . Imagine they were people, standing shoulder to shoulder, and you are looking down on them from up high. What you see are the peoples' heads. What would happen if person 1 suddenly got pushed towards the right, into person 2? Click the "Play" button to find out.
Person 1 knocking into person 2 creates something of a "chain reaction," because the people are all so close togehter. 1 pushes into 2. While 1 recovers, 2 pushes into 3. While 2 recovers, 3 pushes into 4, and so on. What we get is something like "the wave" sporting fans do in a stadium. The "pushing" appears to travel rightwards, down the line of people.
Can you see "the wave" this produces? It travels from person 1 (who is pushed first), rightwards, all the way to the end of the line of people. The wave stops when the last person recovers, and the wave ends.
So what does this have to do with what sound is? Well, imagine that the red dots are air molecules, and the wave is able to spread out more...