Mantra is a sacred sound. It may or may not have any literal meaning. Yaska acharya defines it as 'Mananaath Mantrah' -mantra is that which is thought about. So it is a thought. A charged thought which when repeated will protect one from the limitations of mind & sensory organs. With repeated chanting, one's real nature will be recognized.
Mantras can be classified into Four.
1. Sidha mantra : that will be successful when repeated the required number of times given in the shastra/Kalpa.
2. Susidha mantra : that which will grant success when chanted half the required number of times given in the shastra/Kalpa.
3. Sadhya mantra : that which has to be chanted double the required number of times. &
4. Ari mantra : the mantra if chanted will destroy the Sadhaka. Here the mantra becomes the enemy of the chanter.
So one should chant only after knowing to which class the mantra belongs to. The mantra that suits one should be obtained from a Guru who is capable to find the class to which the mantra belongs.