Perhaps, it’s best to think of mantra as being a cross between poetry and magical incantations. Many mantras don’t make any real or rational sense, even in Sanskrit, and so they can’t really be translated into a normal sentence.

Why to chant Mantra?
Mantras can help us to develop mindfulness. The sound of the mantra is a mental object, and paying attention to the sound of the mantra can be a form of meditation, just as paying attention to the sensations of the breath is a meditation. By bringing the mind back over and over again to the mantra the mind can become more unified and less scattered. We become more attentive and present.

People who learn about the mantra, naturally, would want to know what it means, Why to chant it? How to chant or repeat it? They often ask for a translation of particular Mantra into English or some other Western language. Well, sometimes, I doubt, if the actual meaning is always possible, as the simple word ‘ OM ’, which again is a mantra and has 100 meanings. So, the only true definition is the ‘experience’ which is ultimately created in the one who chants it.

I remember the story of Saint Valmik, the one who was a cruel dacoit, whose only duty was to kill. After realizing his sins, as a penance, he wanted to begin with chanting holy name of ‘Rama’ but couldn’t pronounce the word so keeping an intention of correct word he started chanting ‘Mara’.  He became the noblest sage because of the effect of chanting God's name, though backwards…  MaRa MaRa MaRaMa… RaMaRaMaRaMaRaMa…

In Bhagavad-Gita, God Krishna explains that ‘He’ is the transcendental Om chant and that the chanting of ‘japa’ (chanting a mantra quietly for one's own meditation) is the purest of ‘His’ representations and sacrifices. It is understood that by chanting japa and hearing the holy sounds of the mantra, one can come to the platform of spiritual realization. The energy in the sound vibration of the mantra prepares and opens one's consciousness to higher levels of reality. This is the process of mantra-yoga.

All elements and energies in the Cosmos can be influenced and guided by Mantras.
There are three types of Mantra. Prayer is a way of communing with God. The Guru Mantra represents the essence of prayer, and anchors us in God, the Soul and the Supreme Self. It is the first initiation given by the Master to the disciple on the spiritual path. Bija mantra represents the essence of the Guru Mantra. It is the vibration and the "call" of the soul. It’s effect develop more readily in deep meditation. As it works at the astral level, it guides and influences the course of our destiny.

How we repeat a mantra is also important. The mantra is internalized in five stages: Likhita - through writing…We can write a mantra over and over and over, which is said, fixes it as a powerful impression in our Subconscious mind. ‘Vaikhari’ through speaking the sound which we pronounce. ‘Upamshu’ through whispering…. ‘Manasa’ through thinking.  ‘Japa’ through uninterrupted inner repetition in your meditation.For mantra reciting, a lot depends on the type of mantra you intend to recite and the purpose. Some people recite some mantras daily. It is up to their convenience. The normal number of times a mantra is recited is 3, 11, 21, 27, 54 and 108 or more.

I remember an interesting story of Master and two disciples; one of the disciples was very diligent, though his realization was perhaps not so profound. He set out to accomplish the practice as quickly as possible and recited the mantra incessantly, day and night. After long efforts, he completed his one hundred million recitations, in three years. The other disciple was extremely intelligent, though perhaps not as diligent, because he certainly did not launch into the practice with the same enthusiasm. But when his friend was approaching the completion of his retreat, the second disciple, who had not recited very many mantras, went up on the top of a hill. He sat down there, and began to meditate that all the beings throughout the universe were transformed into an aspect of our own beings. He meditated that the sound of the mantra was not only issuing from the mouth of each and every being, but that every atom in the universe was vibrating with it, and for a few days he recited the mantra in this state of ‘Samadhi’.(Concentrated Meditation)

When the two disciples went to their Master to indicate they had finished the practice, he said, 'Oh, you've both done excellently. You were very diligent, and YOU were very wise. You both accomplished the one hundred million recitations of the mantra.' Thus, through changing our attitude and developing our understanding, practice becomes far more powerful."

We can speak a mantra aloud (chanting) or sing it (bhajan). We can whisper a mantra. We can breathe a mantra. We can repeat the mantra in our spirit. We can repeat the mantra with our attention by thinking it (sumiran). We can repeat the mantra by intention from our attentional principle. We can sense the mantra is already resonating within us, focus our attention upon it and follow the mantra down to profound depths…

There is one mantra for everything and anything, right from blessing a medicine to get world peace,  further go to say there is one mantra for each deity and the deity has his act reflexively as Lord Ganesha is remover of all obstacles and its Mantra is ‘Om Gan Ganpate namah’ , Laxmi is Goddess of wealth and its Mantra ‘Om Kleem Shreem Laxmi’.

There are some of my favorite mantras and one which is said to be the most powerful of all .‘The Gayatri Mantra’ which is the prescribed daily mantra for all Hindus and regarded as the remover of all sins and the bestower of all desired things.

The following is the most commonly recited ‘Gayatri Mantra’.“ Om Bhuh Bhuvah Svah Tat Saviturvarenyam Bhargodevasya Dhimahi Dhiyoyonah Prachodayat”

Om : The primeval sound
Bhur: the physical world
Bhuvah: the mental world
Suvah: the celestial, spiritual world
Tat: That; God; transcendental Paramatma
Savithur: the Sun, Creator, Preserver
Varenyam: most adorable, enchanting
Bhargo: luster, effulgence
Devasya: resplendent, supreme Lord
Dheemahi: we meditate upon
Dhiyo: the intellect, understanding
Yo: May this light
Nah: our
Prachodayath: enlighten, guide, inspire.

The meaning of the Gayatri mantra is as follows: We contemplate the glory of Light illuminating the three worlds: gross, subtle, and causal. ‘I am’ that vivifying power, love, radiant illumination, and divine grace of universal intelligence. We pray for the divine light to illumine our minds.