Mantras are phrases one can chant as an aid to meditation (as a focus). "Om mane padme hum" is probably the most famous and is not actually translatable. You can really use about anything as a mantra during meditation but it's traditional to use phrases that have positive concepts behind them.
The "Om" in the above actually comes from ancient Vedic practice and is believed to be the original sound. Along with "Shanti" (which means "Peace") it gets chanted as a mantra as well.
Om Shanti, Shanti, Shanti.