Noise reduction will remove noise from your signal. Your audio file can be a voice over or mixed song. Most recording devices (analog or digital) have self noise that can use reducing. The better the hardware, the less noise usually needs to be removed. The noise can be random or white noise.   White noise is a random signal with a constant power spectral density. Electronically recorded music usually have a hiss influenced by the voltage of the output signal.

What you get

Run audio through noise reduction tools
Automation on top of compression for extra consistency
EQ to add clarity