Yogapedia states:
"A bija mantra (Sanskrit: बीज मन्त्र) is a one-syllable sound used in meditation or yoga practice.
The Sanskrit phrase bija mantra directly means, “seed mantra” and is comprised of three little words; bija meaning seed, man meaning the mind or to think, and tra meaning an instrument"
"Bija is a Sanskrit term that literally translates to “seed.”
In yoga, this word can be used in a number of ways.
Firstly, bija is a metaphor for the starting point or origin of all creation. It's the seed from which everything originated. Another common use of the term is to refer to bija mantras"
They can be used to cultivate health, vitality and bring down inspiration
Mantra literally means to liberate one's mind
I have also included the Djwhal Khul given sound below the seed syllable