What are Mantras?
Next to breathing (pranayama), mantra is the most important aspect of the Yoga system.
"Man" means mind and "tra" relates to the science of vibration. So we can say that mantra yoga is the scientific use of sound to affect consciousness. These particular sounds help focus the mind and make it one-pointed (ekagra).
The repetition of and contemplation on a mantra restructures patterns in the mind and is a conscious way of controlling and directing the thought-waves of the mind.
In time, as the subconscious mind is cleansed, one is able to enter into states of deep peace, clarity and quietude.

Are Mantras Man-Made?
No, they are not. They have always existed in a latent state as sound energies.
The ancient Sages of India, in profound states of meditation, were able to experience these subtle sound-vibrations of the Cosmos. In time, these were codified in the scriptures, revealed to mankind and came to be known as mantra, the Science of Sound Energy.