Sound is a wave, similar to the ripples on a pond or the ocean waves you might see crashing on a beach. Instead of being a wave on the ocean surface, sound is a wave that travels through air or water.
Because waves move, you can think of a sound wave moving through water. Imagine a tiny section of the water as a particle. This water particle receives a tiny push, then a pull, as sound travels through it. This causes the water particle to vibrate backward and forward around the spot where it was before the sound wave came through. This spot is called its equilibrium position. The movement of the water particle is called vibration.
The water that sound travels through is called a medium. A medium can be anything - a liquid (such as water), a solid (such as the seafloor), or a gas (such as air).
A sound wave is called a compressional or longitudinal wave. This is a picture of a longitudinal wave. The particles in a longitudinal wave move parallel to the direction in which the wave is traveling.
The places where the particles are compressed are regions of high pressure. The places where the particles are pulled apart are regions of low pressure. A sound wave alternately compresses and expands whatever medium it is traveling through.
Water waves on the ocean surface are called transverse waves because the particles move up-and-down as the wave moves left-and-right. The particles in a transverse wave move perpendicular to the direction in which the wave is traveling. You can see this below in the picture of a pond. If a rock is thrown into the pond, it creates a wave. The water moves up-and-down while the wave moves away from where the rock landed. That is called a transverse wave.