According to Yogic Science, a reason why it is so difficult to sit still in meditation is due to an aspect of consciousness sometimes reffered to as Monkey Mind (a step higher - more evolved - then the Reptilian Mind). Its typical character is like a monkey, jumping from here to there being mischeivous when trying to be disciplined. This part of consciousness is doing what it does best which is being vigilantly active and constantly processing stimuli coming in from the senses to report about reality around us.
Mantra is very practical even though its roots are very esoteric and therefor not completely comprehensible. An advanced meditation technique, among many, that if applied correctly will occupy this Monkey Mind enough to settle and tame it. Mantra is a string of words that are repeated throughout the meditative experience. It is said that Mantra repetition is also a form of Asana for the mouth. It is also said that the literal translation of a Mantra is unimportant since it is the sounds that are the real tool at work.
According to Yogic Science the raw primordial sound of the actual letters enunciated (out loud or internally) is the real tool at work here. The sound, without intellectual concepts bound to it, has an influence on the human mind and the surrounding in which this mind is immersed. Since sounds can be thought of as vibration then we can start to sort of understand what these ancients where getting at. The mantra exerts its influence on a quantifiable and (as of yet) non-quatifiable metaphysical level. In the measurable aspect, when recited out loud the mantra is influencing everything with the vibrational frequencies emitted from the vocal chords. On the metaphysical level, when recited to oneself, it is still emitting some sort of frequency but not measurable by current scientific theory since it could be said to be off the measurable part of the the sound spectrum. When you talk, sing, scream, etc.. in your head, there is some sort of activity..and you can hear it in there. The ancients learned this to be a powerful tool to work with the mind.
Note: Before applying mantra to the more evolved mind faculties, the practitioner must relax their body, and the aspect of consciousness called the Reptilian Mind, first. This is done practicing Hatha Yoga (a body conditioning technique) and then through sitting with the head, neck, trunk, spine and pelvis in a straight alingned posture while breathing diaphragmaticaly.
Examples of Mantra:
- So Hum (from the yogic shankacharya tradition) link
- Sat Nam (popular with Kundalini Yoga)
- Hallellujah (for judeo-christian meditators)
- Ribbono shel Olam (according to Rabbi Nachman of Breslov)
- The Hesychastic Jesus Prayer (christian meditators)
- Allah (islamic meditators)
- fnord fnord fnord (Discordians meditatators)
- Hare Krishna, Hare Krishna, Krishna, Krishna, Hare, Hare, Hare Rama, ... (krishnas)