A japa mantra is a word or hymn which bears a spiritual significance; usually, the name of the Almighty or words to praise the Lord.
Japa is the way of chanting or muttering the particular word or hymn repetitively, but, at the same time, so subtly that only the reciter can hear it and nobody else. In fact, according to yogic discipline, to attain the desired spiritual outcome, a practitioner needs to chant the japa mantra internally: that means in his or her own mind. Though, for a beginner, reciting the mantra internally may not come easily; but, through slow and steady effort, a practitioner will be able to do the mental recitation within a few weeks. The word, mantra, is derived from the Sanskrit man, meaning "to think" and the suffix, -tra, meaning "instrumentally." Together, the word literally translates to "instrument of thought."
A japa mantra can be of two types: monosyllabic (i.e. Om or Aum) or a phrase of words (i.e. Aum namah shivaya). According to Hatha yoga, shabda brahman, or the "supreme sound," is the ultimate source of all kinds of sound happening in and around the universe, whether audible or inaudible. But, while each and every sound possesses some great force, no sound is more powerful than a japa mantra. Throughout thousands of years, Vedic people drew words from different Vedic and non-Vedic sources and put the practice to various uses; the most significant is communicating with the Almighty.
On a broader perspective, soulful mental recitation of a japa mantra can result in ultimate salvation or the ultimate spiritual freedom. A sacred hymn works the best if it is given by a guru or other spiritual teacher to a disciple through the process of initiation.
While the mental recitation is the most commonly used path, japa running is also another familiar way of doing it. Japa mala, or a string of 108 prayer beads, is used to recite the mantra. The practitioner must recite the entire mantra once for every bead, counting to 108 for a full circle. Wondering why 108? Well, Hinduism considers 108 a pious number.