The word mantra is derived from two Sanskrit roots: man meaning "mind," and tra meaning "deliverance." A genuine spiritual mantra is, therefore, a transcendental sound vibration that delivers the mind from material anxieties and illusions. The Hare Krishna mantra is universally recognized throughout the Vedic scriptures as the mahā-mantra—the great, foremost of all mantras.
According to the Kali-santarana Upanishad: Hare Krishna, Hare Krishna, Krishna Krishna, Hare Hare Hare Rama, Hare Rama, Rama Rama, Hare Hare iti shodashakam namnam kali-kalmasha-nashanam "These sixteen names are destructive of the evil effects of Kali-yuga. No better means than this is to be seen in all the Vedas."
What makes it the "Great" mantra is its absolute purity, universality, and unparalleled potency. Unlike some Vedic mantras that require strict rules, specific pronunciations, or qualifications like Brahminical initiation to be chanted effectively, the mahāmantra is open to everyone, regardless of caste, creed, or background. Sri Caitanya Mahaprabhu distributed this holy name freely, declaring that it is the prime benediction for humanity.
By chanting the mahāmantra, one is not trying to artificially control the mind by mechanical yoga practices, which are very difficult in this age. Instead, one directly places the mind at the lotus feet of the Supreme Lord. Because it consists entirely of the Lord's holy names and directly appeals for devotional service, it is the most exalted and effective sound vibration for spiritual realization in the current age.