The word Mantra means something
that we will ponder,
that we will reflect upon.
That is, you go to a teacher,
they say something and
I go back home and reflect upon that.

That is the essential feature of Mantra,
to reflect upon again and again.
The purpose of Mantra is to
help us cross a harbour, an obstacle.
That is why the definition of Mantra is:

 मननात् त्रायते इति मन्त्रः॥
mananāt trāyate iti mantraḥ ||
“Who reflects on this,
will cross the obstacle”.

– Extract from an interview with TKV Desikachar on Vedic Chanting