It is a purposeful decision to withhold life sustaining therapy from dying patients (who no longer wish to receive them) so as not to prolong life or interfere with the dying process.
When a DNR order is agreed to, resuscitation is not attempted i.e. advanced cardiac life support, including endotracheal intubation and manual ventilation, chest compression, defibrillation, and cardiac medication. Other therapies which are considered life sustaining like dialysis, vasopressors, blood transfusions, left ventricular assist devices, antibiotics and artificial nutrition can also be withheld.

The patient’s informed refusal to treatment is based on the ethical principle of autonomy and the decision should be taken after a discussion with the physician about the benefits, risks and burdens of the proposed treatment.
When patients are no longer able to speak for themselves due to loss of ability to make decisions, the surrogate decision-maker may be empowered to make the decisions on the patient’s behalf. Surrogates are family members or other intimates.

Fins JJ, Nilson EG. Withholding and Withdrawing Life-Sustaining Care. Hanks G, Cherny NI, Christakis NA, Fallon M, Kaasa S, Portenoy RK (Ed). Oxford Textbook of Palliative Medicine. Oxford University Press, Fourth Edition, 2010, pp.320-329.