Why output impedence should be low?

Why output impedence should be low?
Never heard the word "impedence".

To get all the audio voltage from a source to a target without loss you need voltage bridging, that is a relative low output impedance to a higher input impedance. Usualy the input impedance is at least ten times higher then the output impedance.
An input impedance is called also a load impedance or an external impedance.
An output impedance is called also a source impedance or an internal impedance.