Each deity has its mantras, starting with single-syllable seed or bija mantras and extending to longer mantras, prayers and supplications. Kali’s primary single syllable mantra is KRIM (pronounced Kreem). KRIM refers to Kriya or the power of action, but action of a yogic nature.
KRIM is composed of three primary letters. The letter-k is the first of all the consonants in the Sanskrit alphabet. It indicates manifest existence, power and force. The letter-r is the seed of fire and light. The letter-i is focused energy and will power.
Out of this single seed mantra is developed a longer threefold mantra or three seed-mantras as KRIM HUM HRIM. This has yet more energy and efficacy in awakening the Goddess within us. It begins with the mantra KRIM and builds upon its power.
The mantra HUM (hoom) is composed of two primary sounds. The letter-h is the seed syllable of the element of space or ether. It also represents the sun, Prana and the Purusha principle. The letter-u creates a force field that can both serve to hold in and to push out. HUM represents an explosion of energy, an expression of great power that is pranic, electrical and fiery. HUM is the power of Agni or fire particularly as directed by the wind or Vayu. Whereas KRIM awakens the electrical force or Shakti, HUM serves to direct it with great force, to use it to make great efforts.
HRIM (hreem) is the great mantra of the spiritual heart, hridaya. It is composed of three main letters. The letter-h, as in HUM, represents, space, prana and light. The letter-r as in KRIM represents light and fire. The letter-i as in KRIM represents focused energy or a ray of light, the Shakti as such. Through the mantra HRIM alone one can enter into the spiritual heart and the small space within its lotus (dahara akasha) in which the entire universe is held.
These put together, Kali’s threefold mantra serves to awaken and energize the spiritual heart, hridaya.
The mantra KRIM serves to cut the knots of the heart. It works like a sword. It stimulates the heart energy within us, its primal desire or wish for immortality, love and light.
The mantra HUM gives power to the heart, expanding the energy of prana and Agni (fire) in a strong, if not explosive manner.
The mantra HRIM opens the energy of the spiritual heart which is like the Sun, spreading it into the Infinite.
This threefold Kali Heart mantra can be compared to a kind of spiritual adrenaline. KRIM awakens the energy of the heart, like an electrical jolt to a heart patient whose heart is failing. HUM expands it this current with great force. HRIM stabilizes it as an infinite power and eternal presence.
KRIM draws the Prana from the breath, the sensory and motor organs and directs it into the heart. HUM turns the Prana into a force of fiery meditative power. HRIM connects the individual prana-mind with the power of the Supreme Self, the power of the light of consciousness (Chid-jyoti). This threefold mantra therefore creates a powerful Pratyahara in the yogic sense, it takes our energy back to the spiritual heart.
Excerpt from Article The Goddess Kali and the Spiritual Heart by David Frawley