It is a mantra that has many names and forms. It is called the Rudra mantra, referring to the furious aspect of Shiva; the Tryambakam mantra, alluding to Shiva's three eyes; and its is sometimes known as the Mrita-Sanjivini mantra because it is a component of the "life-restoring" practice given to the primordial sage Shukra after he had completed an exhausting period of austerity. The Mahamrityunjay mantra is hailed by the sages as the heart of the Veda.
Mahamritunjay jaap anustan is done for a long and healthy life and to get rid of prolonged sickness. specially for those on their death bed.
Meaning Of Mahamrityunjay Mantra : It is important to understand the meaning of the words as this makes the rucurrence meaningful and brings forth the results.
OM is not spelt out in the Rig-Veda, but has to be added to the beginning of all Mantras as given in an earlier Mantra of the Rig-Veda addressed to Ganapati.