Mantra is a sound, a word or a phrase which is repeated again and again. It is a powerful sound or magic vibration which creates a significant change in attitude and mind set. It helps in focusing one’s energy. Mantra japa is an indirect way to calm mind. After one thought leaves mind and another thought enters, a mantra is slipped between that gap and slowly that gap is increased by increasing the number of mantra. As one experience deeper meditation all worries and doubts evaporate, mind becomes still and feels the oneness with God and experiences supreme peace. Mantra japa is a very powerful tool to enter Samadhi.
(Excerpt from Japa Meditation - The Easiest Way to Calm the Mind by Chandrika Shubham)