Mantras are special sounds which may or may not having meaning.
In Sanskrit, it is said, "Manana trayate iti mantra".Means mantra is that which liberates you .Mantras are rhythms of the consciousness. Mantra produces vibrations .Through regular, constant reciting mantra you are in the position to produce certain level of vibrations,energy field with in you and through all of yours physical,emotional, and intellectual self.Mantras are not merely the arrangements of words but it is a sound ,vibrations energy bundles mystic in nature.These sounds of mantra harmonies you.
Now modern science also accept the signification of sound energy.One form of sound is noise which provides stress and disturbance to society while another form of sound is music which sooth our nerves and brings peace and harmony in society,ultra sound,x rays are another forms of sound which modern medical science is singfor diagnosing and curing various disorders ,diseases etc.
By moderate use of sound energy physical,mental,emotional and intellectual capacities can be increased.
Ancient yogis/sages revel the knowledge of sound/mantra,in there deep meditation level , they heard some mystic sounds or mantras representing a particular aspect of divine vibrations. Through generations of spiritual gurus this knowledge of mantra have handed down to there aspirants.
Ancient yogis/sages developed the science of mantra yoga,through practice of mantra yoga aspirants can achieve the highest goal of yoga.Significance of mantra chanting are not only accepted by Indian Sanskrit/culture but also various religion like catholic,Buddhist,Sufi, Jain etc accept the role of mantra chanting in there spiritual path. Mantras are like the seeds of powerful energy.By regular reciting mantra aspirant generate a defense shield around him to protect him from bad energy field and lead him towards real knowledge,liberation etc.Mantra works on the principal of repetition,through repetition of mantra aspirant clear his/her Chitta(Mind stuff) and develop sattivic/divine nature. There are many kinds of mantra some are open mantra and some are secret mantra and can be practice only after initiation by guru. All mantras have different vibrations energy,it is up to aspirants what kind of practice he wants with mantra yoga .but one thing is sure it is one of the easy ans secure way to have concentration and meditation at present time .AUM is also a mantra any one can recite it .