Sound is the make up of fluctuating pressure wave in a physical medium, most of the time that medium is air. This is a type of kinetic energy called acoustic energy. If we take a speaker and play audio out of it, we can see the diaphragm of the speaker moving in and out. The same pressure that is moving the speaker moves the air molecules. The air pressure compresses and then decompresses. when this compression and rarefaction happens the energy decreases over a distance. This is what is called the the inverse square law. This law applied to acoustic energy means that the perceived loudness of the sound is inversely proportional to the square of the distance from the source of the sound. If you were standing 4 times the distance to the source, what you would hear would be 1/16th of the energy of the original source. Meaning the further away from the speaker the quieter it is.
Lets look at how we measure and hear sound. We measure acoustic energy in two ways, Amplitude and Time. If we look at a wave form it is a visual representation of energy over time. This graph will show you the examples I have stated.
Time is represented as cycles per second or frequency, one cycle is represented in the graph above as reaching the peak compression and peak decompression then back to original position. These cycles are measured in Hertz. The more cycles per second the higher the pitch is that we perceive. Amplitude is the amount of variation between normal air pressure and the peak compression and decompression, this is measured in Decibels or dBs for short. the larger the variation the louder we perceive the sound. I hope this can help explain what sound is and how we understand it as we hear it.