‘Next of kin’ refers to your closest living relative - either through blood or marriage. In Australia, it usually means a spouse or de facto partner however if you are unmarried or single then your next of kin is your closest living adult relative. This person is the first point of contact if there is an unexpected emergency.
The phrase is predominantly used on legal documents like liability waivers and Wills. When someone becomes incapacitated or dies their next of kin is contacted to make important decisions or carry out certain responsibilities. Depending on the situation they may need to make medical or legal decisions or administer the deceased’s estate.