I’ve only ever used one mantra — which I got from the 1 Giant Mind app: “Aham”. The way I understand the mantra, is it’s basically a sound that settles your mind so that you can spend time in your unconscious. The mantra shouldn’t be a word that you know the meaning of. Rather, it should be more of a sound that works for you.
For TM, that’s part of the long training process — getting a specific mantra, because apparently, there is something to the vibration of the right mantra that can help your practice.
I hope that was helpful.