Pulseaudio and pipewire are kind of like audio drivers (not exactly but anyway). Let’s say you’re running spotify and discord. They both send their audio to pipewire. You can then use pipewire to control how loud each one is. It also supports more complex use cases like if you’re streaming, you can hook up the spotify output and your microphone mixed together into discord’s input so that the mixed audio will be streamed.
Pipewire is newer and basically replaces another system called jack that did the same thing for pulseaudio.