Hare Krishna, Hare Krishna Krishna Krishna Hare Hare,
Hare Rama, Hare Rama Rama Rama Hare Hare.

The maha-mantra, or Hare Krishna mantra, is a prayer addressed to the Supreme Personality of God. Perhaps the earliest mention of the Hare Krishna mantra is in the Kalisantarana Upanishad
mantra is a certain combination of sounds, vibrations that can stop the mind. In translation from Sanskrit, "Man" means mind, "tra" means liberation. So, a mantra is the repetition of certain sounds that give liberation from worry, from faults, from resentment, from irritation, from anger. "Maha" - Big, great, mighty. "Maha mantra" - A powerful mantra.
Kali, invoking the name of God, is the most effective way to purify and build a relationship with God.

“The holy name, the holy name, the holy name of the Lord is the path of spiritual development for those who live in the age of Kali. Not any other way, not any other way, not any other way.

(Chaitanya Charitamrita adi lila 7.76)

Kali-yuga is an ocean of evils, but besides them there is another great virtue: simply by chanting the Hare Krsna maha-mantra one can become liberated from material bondage and enter into the transcendental world.

(Srimad Bhagavatam 12.3.51)
The maha mantra consists of the three Names. These three words - Hare, Krishna and Rama - are the transcendental seeds of the maha-mantra, which is the spiritual call of the conditioned soul pleading to the Lord and His inner energy Hara for protection.

The repetition of the maha-mantra is like the unpretending cry of a child calling to its mother. Mother Hara helps the devotee to gain the favor of the supreme father, Hari, or Krishna, and the Lord reveals Himself to the devotee who sincerely repeats this mantra.
"He sings the maha-mantra Hare Krishna, consisting of the names Hari, Krishna and Rama."
"One who venerates Sri Chaitanya, worships Him and remembers Him - is freed from all sins. He is purified, realizes the Absolute Truth and comes out from under the power of suffering. And there is no other way to attain such perfection."

"Light-skinned but dark within (Krishna), I shall take birth from the bosom of Shachidevi. Accompanied by My companions, servants and weapons, I will play the role of an ordinary man."
"Hara" or "Radha" represents the manifestation of the feminine energetic aspect (in the vocative case Hara is changed to Hare).

"Krishna" is the innermost name of God, translated as the all-attractive one.
"Rama" in the mantra can refer either to Rama, one of Vishnu/Krishna's avatars, or to Balarama, Krishna's brother in his earthly games.
Srila Prabhupada's translation of the maha mantra is: "O all-pleasing and attractive Lord, O inner pleasure energy of the Lord, engage me in devotional service to Himself."