The intent of the mantra is to have NO meaning - to assist us achieving a meditative state of "blank" mind. It truly doesn't matter if they mean something in Sanskrit, Mandarin or Eskimo language and "who believes in what".
The common thing for all mantras are two types of SOUNDS: open low frequency sounds "o", "u" or "a" that resonate with the lower parts of our body and NASAL sounds like "m", "n" that resonate within the head.
You can sing your favorite Bach cantata using these two types of sounds instead of words - after sufficient number of repeats the result will match the best mantra.
Try to "IMAGINE" chanting your mantra in your MIND, rather than aloud. The effect on your MIND should be quite similar - entering a meditative state. The benefit of learning to do it is that you can do it anytime anywhere in public, or when someone tries to upset you.