A more thorough explanation:

A directive to physicians, also known as a living will, health care directive, or advance directive, is a legal document that allows an individual to make decisions about their medical treatment in advance. This document can prevent medical personnel from using life-sustaining treatments for life-threatening health situations. It can also transfer the authority of deciding whether or not to use life-sustaining health treatments to another individual if the person becomes incapacitated.

For example, a person may create a directive that states they do not want to be put on a ventilator if they are in a coma. Or, they may specify that they do not want to receive CPR if their heart stops.

The requirements for creating a legally recognized directive vary from state to state. Some states require multiple witnesses to the signing of a directive, approval by a physician, a specific form, and sometimes even requiring the directive to be notarized. Others may even accept an oral directive. In all states, the person creating the directive may decide at any time to change their mind and receive medical treatment, and a person may make adjustments to the directive later which in some states will require following formal procedures.