This mantra is addressed to Lord Shiva and is taught in the Rig Veda (7 mandala 59 Chapter) as well as the Yajur Veda (3-60). After the penance Lord Shiva taught the Mahamrityunjaya mantra to Sukracharya, who under very compelling circumstances had to teach this to the son of Brihaspati and that is how the devas also got the mantra. This mantra was given (sruti) to Vasistha Maharishi for the welfare of this world. The mantra and explanation given by Sukracharya to Rishi Dadhicha when the latter's body was cut and thrown by Raja Kshuva is recorded in the Shiva Purana.
The name Mrita-Sanjivini mantra has a story behind it. It is said that Sage Sukracharya accepted a challenge of Lord Indra and took up a rigorous penance of hanging upside down from a tree with his face being fanned with fumes of a fire directly beneath his hanging body. And after Sukracharya did this for Vimsottari dasa period (twenty years), Lord Shiva appeared before him and gives him this Maha Mrityunjaya Mantra to restore his physical condition. Hence it also called the name Maha Sanjivini Mantra.
Sukracharya said "The first pada is “trayambakam yajaamahe” and means - we worship or sing the praise of Lord Trayambaka. Tryambaka is the name of Lord Shiva as the father of the three worlds - bhu, bhuva and svarga lokas. He is the father and lord of the three mandala's - Surya, Soma and Agni mandala. He is Maheswara, the lord of the three Guna's - Satva, Rajas and Tamas. He is the Sadashiva, the teacher of the three tatvas - Atma tatva, Vidya tatva and Shiva tatva. He is the father (cause and source) of the three energies (agni) - Aavahaniya, Garhapatya and Dakshinagni. He is the father of all physical creation through the three murti bhuta - Prithvi (solid), Jala (liquid) and Tejas or agni (energy). He is the lord of the three heavens created by the dominance of the three Gunas - Rajas (Brahma), Satva (Vishnu) and Tamas (Shiva). Know Him to be the nirakara (formless) Sadashiva as He is above this physical mode and is their Maheswara. This is the first foot of the mantra (composed of eight syllables)."
"The second pada of the mantra," continued Sukracharya, is “sugandhim pushtivardhanam” Sugandhim refers to the fragrance of the flower that spreads in all directions, and in a similar way Shiva is present in the entire creation, both animate and inanimate. In all the bhutas (modes of existence), in the three Gunas (nature of creation as being Satva, Rajas or Tamas), in the ten indriyas (five gyana-indriyas or senses and five karma-indriyas or organs of action), in all the devas (33 devas are the source of all illumination and enlightenment) and the ganas (hosts of demi-gods), Shiva exists and pervades as the illumine atma (soul) and is their essence. Pustivardhanam is now being explained. That inward dwelling spirit (atman), the Purusha Shiva is the real sustainer of Prakriti (and not vice-versa as all people perceive). Starting with the mahatatva (primordial state of matter/energy) to the individual parts of creation, the entire sustenance of the physically created beings (both animate and inanimate) is done by the imperishable Purusha. You, I, Brahma, Vishnu, the Munis and even Indra & devas are maintained/sustained (by the atma and that is Him). Since the Purusha (atma - Shiva) is the granter of sustenance to prakriti (body/nature), he is 'Pusti-vardhana'."
Having explained the first two pada of the mantra, Sukracharya continued to explain the remaining two pada. He said " the next two pada (consisting of sixteen syllables) is “Urvaarukamiva bandhanaan mrityor muksheeya maamritaat” meaning - Prabhu! Just as the ripe cucumber is severed from the bondage of the creeper, in the same manner may we be delivered from death for the sake of immortality (moksha). Rudra deva is like amrita (nectar of immortality). Those who worship Him with good karma, penance and repentance, meditation, contemplation, prayer or praise, will surely renewed life and vigor. The strength of truth force (in this mantra) is such that Lord Shiva shall definitely free the worshipper from the bondage of death because Shiva alone is the giver of bondage and moksha.
This is the Mritasanjivani mantra and has the power to give back life and rescue from death and great evils. You should adore Lord Shiva and recite this mantra. Water sanctified with this mantra should be drunk all the time. [Authors note: It is well known that the rudraksha bead is placed in a glass of water after being held in the palm during the recitation of the mantra. this water is drunk during the day.]
Mrityunjaya (Bija) mantra
The Bija mantra is given by Kahola Rishi; Haum, Joom, Sah
“Aum Haum Joom Sah
Aum Bhur Bhuvah Swaha”
The mantra is in Gayatri channdah; The mantra devata (deity) is Sri Mrityunjaya (form of Shiva). This mantra is to be used for meditation and at all times for protection from all evils.