How easy it is for sound to travel through a medium is measured by the acoustic impedance. In air, it is relatively easy for sound to move, and acoustic impedance is very small (0.0003 x 10^5). In water, which is more dense, it is more difficult for sound to move, and the acoustic impedance is much bigger (1.5 x 10^5). But what does impedance mean, in terms of the molecules of air and water?
Let's say we have a speaker, which happens to be shaped like a square. When this speaker expands, it pushes one molecule thick square of molecules away from itself. This square bumps into the next square of molecules, which then bumps into the next molecules, and so on, causing the sound to move through the air.
Now let's imagine those squares of molecules as sumo wrestlers. In some media, like air, molecules will be easy to push over into the next row of molecules. These media have a low acoustic impedance, and sound is transmitted through them easily.
In some media, like water, the molecules will push back a little. Media where the molecules resist being pushed have a higher acoustic impedance (as I said, water is 1.5 x 10^5 rayls, 5000 times that of air!).
And in some media, like rock, it will be very difficult to move the next row of molecules. These media have a VERY high acoustic impedance. The acoustic impedance for rock is 4-5 x 10^5 rayls), which is 13,000 to 50,000 TIMES higher than for air!
I thought the sumo wrestlers were a fun way to illustrate impedance. :)