@sc the difference in loudness between quiet and loud parts in the mix. when you hear a drum kinda *kick* in a noticeable way, that's because it's loud in that very short period of time. more to explain than fits in a toot or that i know, but this is a good demonstration of it:

https://www.youtube.com/watch?v=-v6ML2DsBfA

i also recommend looking up the loudness war