Meaning Of Mantra :

The definition of the word "mantra" : "mananat trayate iti mantrah" (that which protects us by being repeated and meditated upon).
"Tranam" means to protect. All of us is familiar with the words in the gita: "paritranaya sadhunam" (to protect the virtuous). "Mananam" means repeating, turning over something in the mind. There is no need to vocalise the words of the mantra. Even if it is repeated mentally, healthy vibrations will be produced in the nadis. If the same --the Vedic mantra -- is chanted loudly ("Vedaghosa") it will give divine joy to the listeners even if they do not understand the meaning. Such a sound has the power to make mankind happy.

Mind, speech and body are dedicated to the Vedas when you mutter a Vedic mantra mentally and vocalise it outwardly during the performance of a rite involving the body. Of the Vedic rites of this kind yajna is the most important.
What is a yajna? It is the performance of a religious duty involving Agni, the sacrificial fire, with the chanting of the mantras. The word itself is derived from the root "yaj" meaning "to worship", to evince devotion. The performance of a yajna is meant to please the Paramatman and the various deities.