In sanskrit, the word "man" means "mind" and the word "tra" means "technique". So, Mantra means "mind-technique".
Mantra Yoga (sometimes called "japa") is the quiet or mental repetition of a meaningful word or phrase that will bring about a positive change within the individual (body, heart, mind). Some people believe that words from more ancient languages, such as Hebrew, Latin, Pali, or Sanskrit (to name but few), have a more powerful influence towards the aim, but I feel that anything that has meaning to you will be just as powerful (even more so because you understand the meaning).
MANTRA AS CONCENTRATION
In the Eight Limbs of Yoga (instructions for success in Yoga), we find that the sixth limb is DHARANA (concentration). There are many forms of concentration and anything harmful and healthy will do (for this matter, spiritual subjects are even better).
We can allocate Mantra Yoga as belonging to Dharana. By single pointedly focusing on the mantra, we are training the mind in concentration.
- "Harmless, simple, easeful, peaceful"
- "Happy, peaceful, content, and satisfied"
- "Virtue, wisdom, peace, love, and Self-Realization"
- "May all beings be happy and free from unnecessary suffering" (some suffering is needed for growth)