A widely known saying from the Vedas states that "Speech is the essence of humanity." All of what humanity thinks and ultimately becomes is determined by the expression of ideas and actions through speech and its derivative, writing. Speech brings everything to life. Ideas remain unactualized until they are brought to life through the power of speech.

Similarly, The New Testament, Gospel of John, starts "In the beginning was The Word. And the Word was with God and the Word was God..."
In mainstream Vedic practices, most Buddhist techniques and classical Hinduism, mantra is viewed as a necessity for spiritual advancement and high attainment. In The Kalachakra Tantra, by the Dalai Lama and Jeffrey Hopkins, the Dalai Lama states, "Therefore, without depending upon mantra...Buddhahood cannot be attained."

Clearly, there is a reason why such widely divergent sources of wisdom as the Vedas, the New Testament and the Dalai Lama speak of common ideas. Here are some important ideas about mantra which will enable you to begin a practical understanding of what mantra is and what it can do.
We know for a fact that every entity in the universe is composed of energy. Every syllable of a word that is let out by us is a carrier of energy and can be used to tap other specific forms of energy in the atmosphere. Mantras are incantations hailing the Gods thereby stimulating the necessary energy flow.

This is mentioned in the following shloka:
Daivadheenam Jagatsarvam Mantradheenamtu Daivatam
Meaning that, the whole universe is ruled by the Devas and the Devas are under the power of Mantras.
The deities are also a part of this energy world, Shiva being the supreme source conscious form and Shakti of energy form. Different energy forms called the Panchabhootas include, the ether element, the air, water, fire and the earth element. The existence of these elements in the human body and as well as in the atmosphere facilitates the interaction of energy between them.

Malfunctioning of the body system is caused by disproportionate energy levels in the system. Each organ has its own consciousness, and is capable of emanating and absorbing a certain amount of energy. Impropriety in this process (of radiation and absorption) can lead to abnormal activity of the organ or the organ system and can manifest as a disease.
But the dysfunctional can be healed with the power of Mantras.