After Om and Aim, Hrim (pronounced 'hreem') is probably the most commonly used bija mantra. It is composed of the sound-Ha, which indicates prana, space and light, with the sound-Ra, which indicates fire, light and dharma, and the sound-I, which indicates energy, focus and motivation.
Hrim is the prime mantra of the Goddess in all of her three main powers of creation, preservation and destruction, and so is the main Shakti mantra, with a more specific application than Aim. It is said to be the Shakti bija and to be equivalent to Om for the Goddess. More specifically, it relates to Parvati or Shakti, the consort of Shiva, who is the destroyer/transformer among the trinity of deities.
Hrim is a mantra of magical force, captivation and empowerment. It can be used relative to any deity or object whose presence we wish to access at the level of the heart. It brings about joy, ecstasy and bliss. Hrim both purifies and exalts us in our inner quest, humbling us before the Divine power so that it can enter into our hearts.
Hrim is a specific mantra for the heart (hridaya in Sanskrit) on all levels, whether the spiritual heart, the heart chakra, the emotional heart or the heart as a physical organ. It energizes the heart, provides warmth, and allows for both deep feeling and deep thought.
In terms of Ayurveda, Hrim can be used to counter heart disease of all types, aiding in longevity and rejuvenation. More specifically, it helps promote the pranic and functional aspects of the heart with it the power of circulation of the blood, the lungs and the nervous system. It has generally Pitta (fiery) energy but some Vata (air) energy as well, stimulating the higher pranas and emotions. In terms of Vedic astrology, Hrim relates to the Sun, the planet of the heart, and helps promote solar energy, expression and charisma.
Hrim is usually a soft mantra but can also be harsh. As a soft mantra, it increases the finer energies of the heart. As a harsh mantra, it can be used to hypnotize or captivate, to dissolve or carry away.
~Dr. David Frawley
Mantra Yoga and Primal Sound
(Secrets of Seed Bija Mantras)