What is a backpack in enlisted?

Could you please explain what is meant by the term 'backpack' in the context of military enlistment? Is it a specific piece of equipment or a more general term used to describe a soldier's personal belongings? I'm curious to know if it serves any particular purpose or if it simply refers to the items a soldier carries with them on deployments. Additionally, is there a standard issue backpack for enlisted personnel, or do soldiers have the option to use their own?