I have seen the reference of ह्रीं at multiple mantras.
What is the exact significance of this mantra and how powerful is this?
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Taken from Siddha Kunjika strotram:
ऐंकारी सृष्टि रूपाये ह्रींकारी प्रतिपालिका क्रींकारी कामरूपिण्यै बीजरूपे नमोस्तुते
ह्रीं is a seed(बीज) mantra of goddess Para-shakti/Durga or Maya.
ह्रीं is symbolization of Maya for her act of guarding and maintaining the existence of the world.
As it is a seed mantra, one can find it's usage in many other mantras, Tantra & Yantra sects.
Reference reading: Siddha Kunjika strotra, Devi Bhagwat(Durga Saptshati) - Markandeya Purana
For more details:Hrim, Hrīṃ
...This Âtman again, united with Mâyâ, composed of desires and Karmas, wants to create, due to the want of discrimination, the twenty-four tattvas, according to the previous Samskâras (tendencies), time and Karma. O Mountain! The re-awakening after Pralaya Susupti is not done with Buddhi (for then Buddhi is not at all manifested). So this creation is said to be effected without any Buddhi (proper intelligence). O Chief of the Immovables! The Tattva (Reality) that I have spoken to you is most excellent and it is my Extraordinary Form merely. In the Vedas it is known as Avyâkrita (unmodified), Avyakta (unmanifested) Mâyâ S'abala (divided into various parts) and so forth. In all the S'âstras, it is stated to be the Cause of all causes, the Primeval Tattva and Sachchidânanda Vigraha. Where all the Karmas are solidified and where Ichchâ S'aktî (will), Jñâna S'aktî (intelligence) and Kriyâ S'aktî (action) all are melted in one, that is called the Mantra Hrîm, that is the first Tattva... - Devi Gita 2.1-50
...The letter "Ha" means gross body and the letter "Ra" means subtle body and the letter "Î" means the causal body; the (dot over the semicircle) is the fourth "Turîya" state of Mine. Thus meditating on the separate differentiated states, the intelligent man should meditate on the aforesaid three Vîjas in the Cosmic body also and he should then try to establish the identity between the two. Before enteriing into Samâdhi, after very carefully thinking the above, one should close one's eyes and meditate on Me, the Supreme Deity of the Universe, the Luminous and Self-Ellulgent Brahmâ.- Devi Gita 4.41-44