How do compact discs work?

So how does a CD hold music and how does a CD player read the music encoded on a CD? Also, let's say that there's a song on a CD that has a a basic drum beat in the background, it starts off quiet and then gets louder, how does the CD tell the CD player to make the sound louder if it is the exact same drum beat, meaning that the bits needed to represent the drum beat don't change? What accounts for the louder sound?