A mantra is a sound, syllable, word, or group of words that are considered capable of "creating transformation.  Their use and type varies according to the school and philosophy associated with the mantra.  

Simple sound mantras can be spoken for good health.   Different sounds or syllables produce vibrations that can stimulate and vibrate different parts of the body stimulating the production of hormones.  

Mantras can also be affirmations.  The group of words are usually formulated in such a ways as to have the biggest effect on the unconscious mind that governs our actions and manifests our life more than our conscious mind. 

Mantras spoken with a faith or an intention of reaching a goal or to manifest something repeated many times will shift the subconscious mind which is more powerful than conscious mind.  The more times a mantra is repeated the more powerful and coherent the intention and faith becomes in the subconscious mind.  The universe than responds to this faith and intention.  Mantras can transform minds, physiology and how we interact with our environment

The world is made up of sound, because sound is vibrating energy.  Many mantras are spoken in the ancient sacred languages in particular Hebrew and Sanskrit.  These mantras spoken in these languages have a more concise effect on our subconscious because these the syllables of these mantras are most harmonious with the sound and energy of our bodies and the universe.  Mantras spoken with the body in a lotus position, with the mudra most harmonious to the mantra, in divine sacred space preferably in a fractal structure that electrifies the space, during a time when the alignment of the cosmos in favorable to your desire, with complete emptiness of being with all focus and concentration of being on the mantra and the intention will manifest what the heart desires.  

Affirmation mantras are designed to program the subconscious mind that permeates the entire body with its epicentre located in the heart.  The heart center is the most powerful center of the body and it is the biggest receiver and giver of information.  However the aura or the light of our being that illuminates out into the world contains the information that the mantra has programed into our subconscious.   Mantras reprogram the subconscious mind and dissipate negative habits or patterns which represent negative energy in our auras.  

Hypnotherapy works with the subconscious mind with suggestion.  The subconscious mind can either except that suggestion and make a holistic change in being that resonates throughout that one person energy field, or not.  The affirmation mantra works the same way.  However with the vast number of repetitions the suggestion is not only accepted it is cemented in the essence of the person's being.  The more the mantra is said the faster the suggesting is cemented and accepted.  The subconscious mind is what manifests things in life.  With our conscious will and intent we can reprogram and clean out our subconscious mind so we can manifest and create the things we want in life.  

The sounds and formulas of mantras are used to stimulate and nourish our organs, or to activate energies within ourselves that can be beneficial for prosperity, wealth, and love.  The formulas of the mantras coupled with faith and intention can also reprogram our conscious mind.  Many traditions have mantras with all sort of functions ie, to dissipate negative karma, to find true love, to find wealth etc. 

Mantras are a tool used for creation.