In Hinduism and Hindu mythology, Mantra means a group of words. There are many such groups of words. Some mantras have meanings. However, there are many mantras which are just syllables, with or without any words. Such mantras do not carry meanings. From the ancient times, Hindus, as well as in many cases, texts of Buddhism and Jainism contain many mantras.
Traditionally, people associate the mantras with various powers. However, it is more a matter of individual faith and experience than a fact of science.