Sound is vibration through a medium such as air or water. This vibration causes a mechanical pressure and displacement of particles, the frequency of which affects the pitch and the amplitude of which affects the volume. A good analogy for sound is dropping rocks into a pond. A big rock makes large ripples (waves) and travels further. The smaller rock makes smaller ripples (waves) and doesn’t travel as far. Sound travels at 330mps.
There was once a bloke called Alexander Graham Bell who you may know invented the telephone? Well he also invented a unit of measurement called the Bel. A 10th of a Bel is a Decibel. A Decibel or dB is used to express the difference between two values – often a measurement and a reference value.