An Enduring Guardian
is someone that makes important medical and lifestyle decisions for you if you are not able to make them for yourself. For example if you are in a coma or go insane.
If you appoint an Enduring Guardian before you lose your mental capacity you will have the opportunity to let them know exactly what your wishes are - you can also do this via a Living Will
If you don't appoint an Enduring Guardian and one becomes necessary the Guardianship Board (in your state or territory) will appoint one for you.
If you want someone to make medical or lifestyle decisions while you are incapacitated then you should make a Living Will
or appoint an Enduring Guardian